ScienceWeek

SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy makers.

August 13, 1999 -- Vol. 3 Number 33

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We are a scientific civilization. That means a civilization
in which knowledge and its integrity are crucial. Science is
only a Latin word for knowledge... Knowledge is our destiny.
-- Jacob Bronowski (1908-1974)

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Contents of This Issue:

1. A Call for Intensive Basic Energy Research
2. On Nucleon Spin
3. On the Possibility of Rapid Climate Change
4. An Inverse Theoretical Approach to the Design of New Materials
5. Notch Signaling and the Control of Cell Fate
6. Structural Rearrangements in Potassium Channel Gating

In Focus: The Layered Earth

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1. A CALL FOR INTENSIVE BASIC ENERGY RESEARCH
It is not often that an individual physicist wears the hats of
both a scientist and a US Congressman. Recently, Rush Holt,
physicist and former director of the Plasma Physics Laboratory at
Princeton University, and now US Congressman from central New
Jersey, made a strong appeal for a "responsible energy future" in
an editorial in the journal _Science_. The author made the
following points:
     1) Affordable energy is the lifeblood of modern society, and
without it the network of transportation, agriculture, health
care, manufacturing, and commerce deemed essential by many of the
world's inhabitants would not be possible.
     2) The truth is that our current system of energy use is
unsustainable and our energy habits will have to change. Although
fossil fuel supplies are limited, total energy use will rise
rapidly in coming years as global economic development continues.
In addition, current *greenhouse gas emissions -- let alone any
greater emissions in the future -- threaten to produce serious
environmental changes.
     3) Other negative effects of fossil fuel pollution, such as
smog, acid rain, water contamination from leaky fuel tanks, oil
refinery emissions, and oil spills, are already very real in many
regions of the globe. Even without global warming, these
immediate problems are enough to warrant change.
     4) For developing countries, cheap, polluting, and
inefficient technologies are often the only available option. The
US is in a position to develop better alternatives, and the US
should take the initiative. But in the US current investment in
research and development in energy is "nothing short of
irresponsible." The US national energy product exceeds US$500
billion annually, yet barely 1 percent of that amount is invested
in research and development.
     5) The author states: "Just as the federal government has a
responsibility to invest now in basic medical research to ensure
the health of present and future generations, so it has a
responsibility to invest now in basic energy research to ensure
both our near-term and long-term economic and environmental
health."
-----------
Rush Holt: A responsible energy future.
(Science 30 July 99 285:662)
QY: Rush Holt [rush.holt@mail.house.gov]
-----------
Text Notes:
... ... *greenhouse gas emissions: The physical basis of the
so-called "*greenhouse effect" is essentially simple: carbon
dioxide gas is transparent to visible light but relatively opaque
to infrared radiation. The same is true of glass. Relatively
high-energy visible light radiation from the sun passes inward
through the atmosphere, warms the surface of the Earth, which
then radiates lower energy in the form of infrared radiation
(heat) back to the atmosphere. But if the atmosphere has a
concentration of infrared impenetrable gases such as carbon
dioxide, the infrared radiation cannot pass out, and the surface
of the Earth underlying the atmosphere cannot cool, and the
surface of the Earth thus will continue to grow hotter.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 13Aug99


2. ON NUCLEON SPIN
The term "nucleon" is a collective term for protons and neutrons,
the main constituents of atomic nuclei. Each nucleon has
approximately the same mass and a spin of 1/2. In this context,
the term "spin" refers to the intrinsic *angular momentum of a
nucleon. According to quantum theory, spin is quantized and
restricted to multiples of h/2ã, where (h) is the Planck
constant. Only integer or half-integer multiples are allowed. The
multiple factor is the "spin number" or "spin state": e.g., for
spin number of + 1/2, the actual spin is + 1/2 x h/2ã. Since
nucleon spin involves a virtual rotation about the axis of the
particle, only two spin states are possible, one clockwise and
one counterclockwise, and in the case of nucleons, the spin
states are + 1/2 and - 1/2. Nucleons constitute 99.9 percent of
matter in the Universe, with approximately the remaining 0.1
percent consisting of electrons. Our current understanding of the
structure of nucleons depends primarily on experiments involving
high-energy impacts of other particles with nucleons, and
detailed analysis of resultant trajectories.
... ... K. Rith and A. Schafer (2 installations, DE) present a
review of current research concerning nucleon spin, the authors
making the following points:
     1) During the past 80 years, experimental study and
theoretical analysis of nucleons have revealed much about these
particles, but certain of their fundamental properties are still
a puzzle to physicists. For the past decade, physicists have
labored to resolve a particular quandary known as the "spin
crisis". This crisis arose in the mid 1980s, when experimental
results indicated that essentially none of the spin of a nucleon
is attributable to the spins of its constituent *quarks. An
intense theoretical effort was then launched to reconcile theory
and experiment.
     2) According to current views of particle physicists,
nucleons contain an incessant "dance" of evanescent particles
flickering in and out of existence. Some of these are *gluons,
the particles that produce the *strong force. The 3 main quarks
that make up a nucleon -- known as the valence quarks -- exchange
gluons back and forth, and the effect is like a strong rubbery
glue that holds them together. Along with the 3 valence quarks
and the gluons, short-lived virtual quarks and *antiquarks
materialize and vanish in pairs, contributing to the nucleon's
properties.
     3) The authors summarize 4 views of a proton:
... ... a) At low resolution, the proton appears to be a "soft"
blob, approximately 2 x 10^(-15) meters in diameter, with a
charge of +1 and an angular momentum (spin) of 1/2.
... ... b) The simplest quark model describes the proton as the
sum of 2 up quarks and 1 down quark, whose individual charges and
spins add up to the properties of the proton. Each quark has a
spin of 1/2, but the total spin will also be 1/2 if, for example,
2 of the quark spins cancel by being oppositely oriented.
... ... c) Experiments at the end of the 1960s revealed quarks to
be essentially point particles within the proton, and the theory
of *quantum chromodynamics (QCD) described the force holding them
together, the force a manifestation of particles (gluons) that
each have a spin of one. The motion of the quarks and gluons
within the proton can also contribute angular momentum to the
proton spin.
... ... d) The full quantum description of quantum chromodynamics
adds a complicated flickering dance of virtual quarks and
antiquarks, including strange quarks not usually considered part
of ordinary matter. The details of how this dance produces the
spin of the proton are still too difficult to be calculated
reliably, and are only gradually being revealed by experiment.
-----------
K. Rith and A. Schafer: The mystery of nucleon spin.
(Scientific American July 1999)
QY: Klaus Rith, University of Erlangen-Nurnberg, DE.
-----------
Text Notes:
... ... *angular momentum: In general, angular momentum is a
property of any rotating or revolving system, its value dependent
on the distribution of mass and velocity about the axis of
rotation or revolution. In general, the angular momentum of a
particle is partitioned into orbital angular momentum and
rotational (intrinsic) angular momentum.
... ... *quarks: In general, a quark is a hypothetical
fundamental particle, having charges whose magnitudes are
one-third or two-thirds of the electron charge, and from which
the elementary particles may in theory be constructed.
... ... *gluons: Quarks are believed to be held together through
the exchange of gluons, massless particles that carry the *strong
force (see background material below).
... ... *strong force: The fundamental forces comprise the
gravitational force, the electromagnetic force, the nuclear
strong force, and the nuclear weak force. The strong force is
approximately 100 times stronger than the electromagnetic force.
... ... *antiquarks: The antimatter quark entity. In general,
antiparticles are homologs of elementary particles but with
opposite charge. The positron, for example, is the antimatter
particle homologous to the electron. Matter composed entirely of
antiparticles is called antimatter.
... ... *quantum chromodynamics: The "chromo-" in chromodynamics
derives from the use of designated "color" attributes of quarks,
the various "colors" labels for various quark properties (see
background material below).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 13Aug99
-------------------
Related Background:
PARTICLE PHYSICS: ON GLUONS AND GLUEBALLS
The physics of elementary particles is in the midst of an era of
conceptual complexity that some people find unsettling and other
people find invigorating. A quark is a hypothetical fundamental
particle, having charges whose magnitudes are one-third or
two-thirds of the electron charge, and from which the elementary
particles that have an apparent internal structure may in theory
be constructed. Quarks are believed to be held together through
the exchange of gluons, massless particles that carry the *strong
force. At the present time, 18 different quarks with various
properties are thought to exist, with a corresponding number of
*antiquarks. Gluons have a "sticking" property -- they can
agglomerate -- and agglomerations of gluons alone are called
"glueballs". ... ... F.E. Close and P.R. Page review current
ideas concerning gluons and glueballs, and the authors make the
following points: 1) Along with fractional electric charge,
quarks also have "flavor" in 6 varieties (up, down, charm,
strange, top, and bottom), and "color" (red, yellow, or blue).
[*Note #1] 2) Quarks may also attach to antiquarks, particles
that have opposite charge, and an antiquark comes in anticolors
(anti-red, anti-yellow, anti-blue). An anticolor is
mathematically denoted by negative color, and a color and its
anticolor attract. 3) The theory of electromagnetism describes
the attraction between opposite electric charges. In the 1940s,
physicists merged electromagnetism with relativity and quantum
theory, creating quantum electrodynamics (QED). This theory --
the most successful theory known to physics -- holds that the
electromagnetic force is transmitted by massless objects called
photons. These quanta of light banish the classical idea of
action at a distance. It can be said that photons bounce between
an electron and an antiparticle (the positron) in such a manner
as to draw the two together. 4) The equivalent theory of color
charges, which communicate via the *strong force, is called
quantum chromodynamics (QCD). Gluons, the massless quanta of the
strong force, transmit the color interactions. 5) Gluons are
fundamentally different from photons. Photons do not have
charge, so one photon cannot push or pull on another photon.
Gluons, however, are themselves colored. A red quark, for
example, can turn into a blue quark by radiating a red/anti-blue
gluon. Basically, a gluon can attract another gluon. Another
difference between photons and gluons is that while photons
uniformly surround electrons, forming a shell with spherical
symmetry whose density falls off with distance, gluons are not
uniformly distributed and instead clump together into a tube
linking a quark and an antiquark. The color originating in the
quark can be thought to "flow" through the tube to the
antiquark, where it becomes absorbed. 6) In 1972 H. Fritzch and
M. Gell-Mann predicted that two or more gluons can combine into
a strongly bound, neutral-colored particle of pure "glue". This
hypothetical object is called a "glueball". A glueball is
thought to have a radius of 0.5 x 10^(-15) meters (less than
that of a proton), and exist for less time than light takes to
cross a hydrogen atom. 7) The authors state that although the
idea of glueballs was elegant, quantum chromodynamics is a
"rather messy theory", since the peculiar "sticky" character of
the strong force makes it impossible to perform exact
calculations. Almost everything known about color and glue comes
not from direct calculation but from massive computer
simulations known as "lattice QCD". 8) Finally, the authors
discuss various current and planned future attempts to detect
the existence of glueballs, and they conclude: "One of these
experiments will, we fondly hope, upturn unambiguous evidence of
unadulterated glue."
----------
F.E. Close and P.R. Gage (2 installations, UK US)
Glueballs.
(Scientific American November 1998)
QY: Frank E. Close, University of Oxford, UK.
-----------
Text Notes:
... ... *strong force: The fundamental forces comprise the
gravitational force, the electromagnetic force, the nuclear
strong force, and the nuclear weak force.
... ... *antiquarks: The antimatter quark entity. In general,
antiparticles are homologs of elementary particles but with
opposite charge. The positron, for example, is the antimatter
particle homologous to the electron. Matter composed entirely of
antiparticles is called antimatter.
... ... *Note #1: In this context, flavors and colors are labels
for specific sets of properties associated with specific types of
quarks. Some people call these labels "whimsical", but perhaps
there is some sense to the whimsy, since it emphasizes that at
the present time we are apparently unable to describe the
properties and behaviors of the fundamental particles with
classical language (i.e., with the language of old models).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 6Nov98
-------------------
Related Background:
EVIDENCE FOR A QUARK-GLUON PLASMA
... According to current theory, quarks and gluons cannot exist
in isolation. But theory also predicts that at temperatures
10^(12) degrees Kelvin or greater (such as existed during the
first 0.01 second of the history of the universe) a drastic
change in the structure of nuclear matter occurs, and only
descriptions in terms of quarks and gluons apply. A plasma is a
gas consisting entirely of equal numbers of positive and negative
charges. ... ... F. Wilczek (Institute for Advanced Study
Princeton, US), in a short review of recent experiments that for
the first time apparently produced a quark-gluon plasma, "an
extraordinary new state of matter", suggests that an important
question is whether, as a function of temperature, the transition
from ordinary matter to a quark-gluon plasma is continuous or
discontinuous. A discontinuous transition could imply explosive
instabilities, which in turn may have been important in the
evolution of the early universe.
QY: Frank Wilczek [wilczek@ias.edu]
(Nature 22 Jan 98) (Science-Week 23 Jan 98)
-------------------
Related Background:
FIRST EVIDENCE OF AN EXOTIC MESON
In particle physics, the term "meson" has had an interesting
history. Between 1939 and 1947, it was used to refer to what are
now called "muons". In 1947, the definition changed, and meson
now refers to an unstable, strongly interacting sub-nuclear
particle that consists of a quark bound to an anti-quark. There
is more than one type of meson, but they all consist of quarks
and anti-quarks bound together by "gluons". In the 1970s,
theorists predicted the existence of "exotic" mesons whose gluon
linkages between quarks and anti-quarks had the mathematical
properties of vibrating strings. But no one had or has yet
provided evidence of exotic mesons. ... ... Now Suh-Urk Chung et
al (Brookhaven National Laboratory, Upton NY US) have apparently
finally identified such a particle in collisions of fast-moving
mesons called "pions" with protons in a liquid hydrogen target.
They have determined the exotic meson's mass to be approximately
1.4 Gev (billion-electron-volts), and another laboratory, using a
completely different experimental method, has evidently independ-
ently confirmed an exotic meson with the same mass of 1.4 Gev.
But whatever excitement there is about finally identifying the
exotic meson, it is apparently combined with puzzlement, because
the revealed mass is less than what had been predicted. Revisions
of theory may be in order, since in the arena of particle
physics, revisions are the usual consequence of puzzlement.
QY: S-H. Chung, Brookhaven National Lab. (516) 344-8000.
(Phys. Rev. Lett. 1 September) (Science-Week 12 Sep 97)


3. ON THE POSSIBILITY OF RAPID CLIMATE CHANGE
Over the course of geologic history, the environment on Earth has
been far from static. Geologic evidence suggests that 600 million
years ago the atmosphere lacked sufficient oxygen to support
animal life. More recently, as indicated by sediments recording
conditions over the past 500,000 years, the climate of the planet
varied between at least two different states. The record from the
past 150,000 years is particularly well-preserved, offering
details concerning repeated climate changes. Between
approximately 131,000 and 114,000 years ago, a warm period
similar to the climate of today occurred. This was followed by
what is called the "Wisconsin ice age", which ended approximately
12,000 years ago when the current relatively warm *Holocene
period began. ... ... Kendrick Taylor (Desert Research Institute,
US) presents a review of the research of a large project to
develop a climate record for the past 110,000 years, the author
making the following points:
     1) The layerings of glacial ice record seasonal variations
of temperature, snowfall, concentrations of atmospheric gases,
and atmospheric circulation patterns. In general, the weight of
accumulating snow compresses the snow below it, trapping
atmospheric gases, dust, and chemicals, and a deep ice core thus
provides a sequential record amenable to analysis.
     2) The author reports that by examining ice cores from
Greenland, he and his colleagues have determined that climate
changes large enough to have extensive impacts on our society
have occurred in a time-frame of less than 10 years. The author
suggests that the climate of Earth could change significantly
during a lifetime, that we are still a long way from being able
to predict such a change, but we are getting closer to an
understanding of how it might occur. A pressing concern is
whether anthropogenic changes in the atmosphere of the planet
might perturb climate stability.
     3) The author points out that climate is the result of the
exchange of heat and mass between the land, ocean, atmosphere,
ice sheets, and space. As long as changes to the land, ocean,
atmosphere, and ice sheets stay below certain thresholds, climate
changes will occur slowly. But climate will change rapidly if
those thresholds are crossed. *Greenhouse warming, for example,
by altering ocean circulation and the flow of tropical heat to
the North Atlantic, could lead to rapid cooling in eastern North
America, Europe and Scandinavia. Altered ocean circulation could
lead to much larger changes. We have no experience predicting
climate switches between stable modes.
     4) The author suggests human ingenuity would most likely
allow us to adapt to a rapid change in climate, but we would pay
a larger price than our civilization has ever known. The author
poses a scenario: "Imagine the economic and social cost of
moving, in a 20-year period, most of our agricultural activities
500 miles south of their current locations. Imagine the social
cost and famine if agriculture could not be relocated quickly
enough."
     5) Although we do not know the critical level of greenhouse
gas concentration that would trigger a rapid climate change, we
do know that reducing the rate of greenhouse emissions would help
in two ways. First, the atmospheric concentration of greenhouse
gases would increase more slowly. Second, numerical models
predict that the climate threshold will occur at a higher
concentration of greenhouse gases if the concentration of
greenhouses increases slowly.
     6) The author suggests it will be another 20 years before
the climate changes that are predicted to be associated with the
greenhouse effect becomes large enough to be unambiguously
differentiated from naturally occurring variations in climate.
As a society we have the choice of ignoring the warning signs or
taking some action.
-----------
Kendrick Taylor: Rapid climate change.
(American Scientist Jul/Aug 1999 87:320)
QY: Kendrick Taylor [kendrick@dri.edu]
-----------
Text Notes:
... ... *Holocene period: The most recent epoch of the geologic
time scale, from approximately 10,000 years ago to the present.
... ... *Greenhouse warming: See notes to report #1, this issue.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 13Aug99
-------------------
Related Background:
ON CLIMATE FORCINGS IN THE INDUSTRIAL ERA
A "climate forcing" is an imposed perturbation of the Earth's
energy balance with space, for example, a change of the solar
radiation incident on the planet, or a change of carbon dioxide
in the Earth's atmosphere. The unit of measure of climate forcing
is Watts per square meter. Thus, the forcing due to the increase
of atmospheric carbon dioxide since pre-Industrial times is
approximately 1.5 Watts per square meter. Climate change is
combination of deterministic response to forcings and *chaotic
fluctuations -- the chaos a consequence of the nonlinear
equations governing the dynamics of the system. Quantitative
knowledge of all significant climate forcings is needed to
establish the contribution of deterministic factors in observed
climate change and to predict future climate. J.E. Hansen et al,
in a review of current considerations concerning climate forcings
in the Industrial era, make the following points: 1) The forcings
that drive long-term climate change are not known with an
accuracy sufficient to define future climate change. 2)
Anthropogenic greenhouse gases, which are well-measured, cause a
strong positive (warming) force. But other, poorly measured,
anthropogenic forcings, especially changes of atmospheric
aerosols, clouds, and land-use patterns, cause a negative forcing
that tends to offset greenhouse warming. 3) One consequence of
this partial balance is that the natural forcing due to solar
irradiance changes may play a larger role in long-term climate
change than inferred from comparison with greenhouse gases alone.
Current trends in greenhouse gas climate forcings are smaller
than in popular "business as usual" or 1 percent per year carbon
dioxide growth scenarios. The authors suggest that a summary
implication of their considerations is a paradigm change for
long-term climate projections: uncertainties in climate forcings
have supplanted global climate sensitivity as the predominant
issue. The authors further suggest that climate forcing scenarios
are essential for climate predictions, but if only one forcing
scenario is used in climate simulations, as has been a recent
tendency, the scenario itself is likely to be taken as a
prediction, as well as the calculated climate change. The authors
recommend that the use of multiple scenarios will aid objective
analysis of climate change as it unfolds in coming years.
-----------
J.E. Hansen et al (6 authors at National Aeronautics and Space
Administration, US)
Climate forcings in the Industrial era.
(Proc. Natl. Acad. Sci. US 27 Oct 98 95:12753)
QY: James E. Hansen [jhansen@giss.nasa.gov]
-----------
Text Notes:
... ... *chaotic fluctuations: The term "chaotic", in this
context, is specific. In the study of physical systems, the
term "chaotic behavior" has a specific meaning: the behavior of a
system is said to be "chaotic" if its final state is so sensitive
to the system's precise initial conditions that the behavior of
the system is in effect unpredictable and cannot be distinguished
from a random process, even though the behavior of the system is
strictly determinate in a mathematical sense. In other words, a
deterministic system characterized by extremely sensitive
instabilities, despite the system being determinate, can exhibit
behavior that is unpredictable, and the system is then called
"chaotic". During the past several decades, the analysis of such
chaotic systems has intrigued both physicists and mathematicians.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 4Dec98


4. AN INVERSE THEORETICAL APPROACH TO THE DESIGN OF NEW MATERIALS
In general, the interplay of theory and experiment in materials
science has usually depended on the use of a real material as a
starting point. The typical procedure is to begin from the real
material, use theory to describe its *energy-well structure, find
minimum energy states, predict the microstructure and, in some
cases, the material behavior, and then compare with experiment.
... ... K. Bhattacharya et al (3 authors at 3 installations, US
UK) present a short review of a new approach in materials
science, the use of theory in a manner inverse to the above
scheme. The new method is to begin with a theoretical concept of
an interesting property or effect, formulate a hypothetical
energy-well structure that produces this effect via energy
minimization or the solution of a dynamic theory, propose a
hypothetical material, then go to the laboratory and actually
make the material. The authors suggest this inverse procedure is
one of the most exciting for theory and for materials science, as
it can lead to an entirely new material that might not have been
anticipated by purely experimental approaches. The authors have
recently followed this inverse procedure for the development of
new *ferromagnetic shape-memory materials. The free energies of
such materials are sensitive to deformation, magnetization, and
temperature. As the temperature is decreased, these materials
exhibit a set of energy wells associated with a ferromagnetic
transition. With a further decrease in temperature, these energy
wells are joined by another set of wells associated with a
*martensitic transformation. All of these wells are positioned on
a complex 12-dimensional energy landscape precisely restricted by
conditions of symmetry and rotational invariance. The central
question for ferromagnetic shape memory is: What energy-well
structures lead to a large change of macroscopic shape when a
magnetic field is applied to the specimen? An analysis of this
question, and its application to a program of alloy development,
have now produced materials that exhibit, under moderate applied
fields, approximately 50 times the field-induced *strain of known
giant *magnetostrictive materials.
-----------
K. Bhattacharya et al: The mathematics of microstructure and the
design of new materials.
(Proc. Natl. Acad. Sci. US 20 Jul 99 96:8332)
QY: R.D. James [james@aem.umn.edu]
-----------
Text Notes:
... ... *energy-well structure: In general, an "energy well" is a
region in a force field in which the energy decreases abruptly,
and on all sides of which the energy is greater. In a 2-
dimensional force-energy diagram, an energy well is a valley
between 2 rising hills. Since energy wells represent energy
minima, they are of some importance in determining the preferred
states and behaviors of the system.
... ... *ferromagnetic shape-memory materials: In general,
ferromagnetic materials are substances showing magnetic
properties similar to those of iron, i.e., high magnetic
susceptibility, permanent magnetism, etc. Such materials include
nickel, cobalt, and many alloys. Ferromagnetic materials are
capable of being magnetized by weak magnetic fields and exhibit
magnetic hysteresis. In these materials, the variation of
magnetization with temperature is such that at a certain
temperature (Curie point; Curie temperature) there is a
transition from ferromagnetism to *paramagnetism. The
characteristics of ferromagnetic substances have been explained
by the presence of domains, regions of crystalline matter
containing atoms whose magnetic moments are aligned in the same
direction. In general, in this context, a "shape-memory" material
is a special alloy which, after being deformed, can recover its
original shape when heated. In ordinary materials, strain beyond
the elastic limit produces permanent deformation. Shape-memory
behavior is inherently connected with a solid-state phase
transformation known as "martensitic transformation", a type of
microstructural change wherein the initial crystal structure
transforms to a new crystal structure without diffusional mixing
of atoms (see below). If a martensitic transformation is
thermoelastic, shape-memory can result.
... ... *paramagnetism: Paramagnetic substances such as liquid
oxygen have a capability to be magnetized which is slightly
greater than that of a vacuum and much less than that of iron.
When placed in a magnetic field, paramagnetic substances are
magnetized parallel to the lines of force of the field to an
extent proportional to the intensity of the field (but not at
extremely low temperatures or extremely high fields). When
removed from an applied magnetic field, the magnetization of
paramagnetic substances returns to zero.
... ... *martensitic transformation: In general, in a martensitic
transformation, below a critical temperature, the unit cell of
the crystal undergoes a bifurcation into unit cells of different
and lower symmetry. In addition to many alloys, ceramics and
proteins also undergo this type of transformation.
... ... *strain: In this context, "strain" is the deformation
produced in a solid as the result of stress (external forces
acting on the system).
... ... *magnetostrictive materials: In general,
"magnetostriction" is the change in length of a ferromagnetic
substance when it is magnetized. The phenomenon involves the
state of strain of a ferromagnetic sample depending on the
direction and extent of magnetization, and has an important
application in devices known as magnetostriction transducers.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 13Aug99


5. NOTCH SIGNALING AND THE CONTROL OF CELL FATE
The construction of an organism from a single egg cell to a
multicellular 3-dimensional structure of characteristic shape and
size is the result of coordinated gene action that directs the
developmental fate of individual cells. So-called "Notch-
signaling", a mechanism conserved in evolution, is used by
multicellular animals (metazoans) to control cell fates through
local cell interactions, and the realization by researchers that
this signaling mechanism controls an unusually broad spectrum of
cell fates and developmental processes (in organisms ranging from
sea urchins to humans) has in the past decade resulted in a large
number of Notch-related studies. The classical identification of
a gene encoding what is now called the Notch receptor was first
made in the fruit fly Drosophila in 1919 by O.L. Moohr, who noted
that certain mutations caused notches at the margins of the fly's
wings. It is now known that the _notch_ gene encodes a protein
(the Notch receptor) essential for the proper differentiation of
*ectoderm, the embryonic cell layer that gives rise to skin and
its derivatives. Expression of the _notch_ gene is apparently
under the control of a set of genes controlling the development
of the nervous system (neurogenic genes). The gene product of the
_notch_ gene is a transmembrane protein (the Notch receptor)
containing 2703 amino acids. In general, in Drosophila, the
_notch_ gene determines which of the equivalent *epidermal cells
of the insect embryo generate skin cells and which cells generate
nerve cells of the peripheral nervous system: the gene apparently
channels a bipotential cell into one of two paths. Soon after
*gastrulation, a region of approximately 1800 ectodermal cells
differentiates into neural cells and skin cells, and the
differentiation choice of individual cells is apparently
controlled by the _notch_ gene. In the absence of the _notch_
gene in the embryo, the cells develop into neural precursors
rather than into a mixture of neural precursors and skin cells,
and the embryos eventually die. In the early 1990s, Heitzler and
Simpson proposed that the Notch protein serves as a receptor for
intercellular signals involved in distinguishing equivalent
cells. ... ... S. Artavanis-Tsakonas et al (3 authors at Harvard
University, US) present a review of current research in Notch
signaling, the authors making the following points:
     1) In developing animals, Notch signaling amplifies and
consolidates molecular differences between adjacent cells. The
implementation of a particular developmental program modulated by
Notch depends, however, on how Notch integrates its activity with
other cellular factors.
     2) In a developing metazoan, many different processes
modulate Notch activity, including direct interactions with Notch
signaling and interference with the synthesis and maturation of
Notch signaling elements.
     3) The fundamental nature of this signaling mechanism and
its ability to influence many specific developmental events in a
context-dependent manner may find medical applications. It is
conceivable that appropriate manipulation of Notch signaling may
become a useful tool in addressing a variety of human dysplastic
conditions, as well as tissue regeneration.
-----------
S. Artavanis-Tsakonas et al: Notch signaling: Cell fate control
and signal integration in development.
(Science 30 Apr 99 284:770)
QY: Spyros Artavanis-Tsakonas, Harvard University 617-432-1550.
-----------
Text Notes:
... ... *ectoderm: In the embryos of animals, there occurs the
transformation of a single-layer *blastula into a 3-layered
gastrula consisting of ectoderm, mesoderm, and endoderm
surrounding a cavity with one opening. The 3 layers are called
the "germ layer", and these layers, via further cell
differentiation and proliferation, determine the development of
all the major body systems and organs.
... ... *blastula: The stage in embryonic development immediately
following the cleavage stages of the fertilized egg cell. The
blastula consists of a hollow fluid-filled ball of cells one
layer thick.
... ... *epidermal cells: The term "epidermal" refers to the
superficial *epithelial portion of the skin.
... ... *epithelial: In animals, epithelial cells (epithelium)
compose the cell layers that form the interface between a tissue
and the external environment, for example, the cells of the skin,
the lining of the intestinal tract, and the lung airway passages.
... ... *gastrulation: In general, the transformation of a
single-layered blastula into a 3-layered gastrula.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 13Aug99


6. STRUCTURAL REARRANGEMENTS IN POTASSIUM CHANNEL GATING
Ion channels are protein channels in cell membranes that allow
ions to pass from extracellular solution to intracellular
solution and vice versa. Most ion channels are selective,
allowing only certain ions to pass, and an individual cell has
ion channels with various ion selectivities. The selectivity of
an ion channel can be "gated", the channel effectively opened or
closed, and ion channels are said to voltage-gated or ligand-
gated, depending on how the change in selectivity is provoked.
The opening of a previously closed ion channel produces a sudden
increase in transmembrane conductance for that ion, and the
process is called "activation". The gating of the movements of
ions through ion channels is of considerable importance for
various processes in all living systems, and forms the basis of
the electrical activity of all nervous systems. Recently (see
background material below), an important advance in ion-channel
research occurred with the experimental determination of the
crystal structure of a potassium channel (KcsA) in the bacterial
species Streptomyces lividans. The structure involves a
tetrameric complex with a centrally located pore framed by the
apposition of individual subunits, each subunit with 2
transmembrane helices (TM1 and TM2) flanking a "selectivity
filter". Intensive studies of this potassium channel in *planar
lipid bilayers have been in progress in a number of laboratories.
... ... E. Porozo et al (3 authors at University of Virginia, U)
now report a study of the structural rearrangements underlying
activation gating in this potassium channel, the study using
*spin-labeling methods and *electron paramagnetic resonance
spectroscopy. The authors report that a comparison of the closed
and open conformations of the channel revealed periodic changes
in spin-label mobility and intersubunit *spin-spin interaction
consistent with rigid-body movements of the two transmembrane
helices TM1 and TM2. These changes involve translations and
counterclockwise rotations of both helices relative to the center
of symmetry of the channel. The movement of TM2 apparently
increases the diameter of the permeation pathway along the point
of convergence of the four subunits, thus opening the pore.
Although the extracellular residues flanking the selectivity
filter remained immobile during gating, small movements were
detected at the *C-terminal end of the pore helix, and the
authors suggest this has possible implications for the gating
mechanism.
-----------
E. Perozo et al: Structural rearrangements underlying
K(+)-channel activation gating.
(Science 2 Jul 99 285:73)
QY: Eduardo Perozo [eperozo@virginia.edu]
-----------
Text Notes:
... ... *planar lipid bilayers: The cell membrane consists of a
lipid bilayer and associated proteins, the ensemble approximately
75 to 100 angstroms in thickness. Similar membranes are also
found within a cell surrounding various organelles. Lipid
bilayers are spontaneously forming self-organizing bimolecular
layers of certain molecules (lipids) with long nonpolar chains
terminated by a polar group. In addition to their presence in
cell membranes, such molecules (surfactants) are also found in
soaps. A variety of artificial lipid bilayer membrane systems can
be investigated in the laboratory.
... ... *spin-labeling methods: A "spin-label" is a synthetic
paramagnetic organic free radical incorporated in a macromolecule
or assemblage of macromolecules and used, in particular, in
electron paramagnetic resonance spectroscopy.
... ... *electron paramagnetic resonance spectroscopy: (ESR) This
technique is used to investigate paramagnetic centers in a
molecular system. Only electrons whose spin is not paired with
the oppositely directed spin of another electron give an ESR
signal. With this technique, information can be obtained about
certain transitional ions, free radicals, and free electron
centers. A probe giving an ESR signal can be incorporated into
membrane lipids or attached to proteins to enable otherwise
inaccessible systems to be studied. Through analysis of ESR
spectra, rates of molecular motion and relative orientation of
spin-labeled molecules whose motion is restrained by surrounding
molecules can be determined. Measurements of rates of molecular
motion and molecular orientation have proved to be important in
the study of a variety of biological problems.
... ... *spin-spin interaction: In this context, an interaction
of two neighboring paramagnetic entities, the interaction
producing a change in ESR signal.
... ... *C-terminal end: In general, this refers to the end of
any polypeptide chain at which the 1-carboxy function of a
constituent alpha-amino acid is not attached in peptide linkage
to another amino acid residue.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 13Aug99
-------------------
Related Background:
ANALYSIS OF POTASSIUM ION MEMBRANE CHANNEL STRUCTURE
... The potassium ion channel from the prokaryotic soil bacterium
Streptomyces lividans is an integral membrane protein with
sequence similarity to all known potassium ion channels,
particularly in the pore region. ... ... Doyle et al (8 authors
at Rockefeller University, US) report an x-ray analysis (data to
3.2 angstroms) of the Streptomyces lividans potassium channel
reveals four identical subunits create an inverted cone cradling
the selectivity filter of the pore in its outer end. The narrow
selectivity filter is only 12 angstroms long, whereas the
remainder of the pore is wider and lined with hydrophobic amino
acids. The selectivity filter is apparently held open by
structural constraints to coordinate potassium ions but not
smaller sodium ions. The authors suggest the architecture of the
pore establishes the physical principles underlying selective
potassium ion conduction.
QY: Roderick MacKinnon (mackinn@rockvax.rockefeller.edu)
(Science 3 Apr 98) (Science-Week 17 Apr 98)
-------------------
Related Background:
SIMILAR STRUCTURE OF PROKARYOTIC VS. EUKARYOTIC K(+) CHANNELS
Toxins from scorpion venom are known to interact with potassium
ion channels in eukaryotic cell membranes. Mackinnon et al (5
authors at Rockefeller University, US) report the use of resin-
attached mutant potassium ion channels from the bacterium
Streptomyces lividans to screen scorpion venom, and the toxins
that interact with the channel were identified by mass
spectrometry. The authors suggest their results indicate that the
prokaryotic potassium ion channel, whose structure has now been
revealed, has the same pore structure as eukaryotic potassium ion
channels, and that this structural conservation, through the
application of their techniques, offers a new approach to
potassium ion channel pharmacology.
QY: Roderick MacKinnon (mackinn@rockvax.rockefeller.edu)
(Science 3 Apr 98) (Science-Week 17 Apr 98)


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 

IN FOCUS: THE LAYERED EARTH
"During the nineteenth century, the nature of the Earth's
interior was a matter of fierce and fascinating debate. All
theories were hampered by a lack of evidence -- the nature of
rocks deep below the surface was unknown. In 1906, Richard D.
Oldham observed that compressional seismic waves (P waves) slow
abruptly deep with the Earth and can penetrate no further. This
was strong evidence in favor of a liquid core. Three years later,
Andrija Mohorovicic noticed that the velocity of seismic waves
leaps from 7.2 to 8.0 km/s at around 60 km deep. He had
discovered the 'Moho' seismic discontinuity that marks the crust-
mantle boundary. In 1926, Beno Gutenberg obtained evidence for a
seismic discontinuity at the core-mantle boundary. This, the
Gutenberg discontinuity, was confirmed during the 1950s when
world-wide records of blasts from underground nuclear detonations
were scrutinized. Subsequent studies of the Earth's seismic
properties, using seismic waves propagated by earthquakes and by
controlled explosions to 'x-ray' the planet (a technique called
seismic tomography), have revealed a series of somewhat distinct
layers or concentric shells in the solid Earth. Each shell has
different chemical and physical properties..."

-- Richard John Huggett: _Environmental Change_
   (Routledge, London 1997, p.56)
[The author is Senior Lecturer in Geography at the University of
Manchester, UK]


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