|
ScienceWeek
CELL BIOLOGY: ON CELL MEMBRANE PROTEIN CHANNELS
The following points are made by J. Benach and J.F. Hunt (Nature 2004 427:24):
1) All cellular proteins are synthesized in the body of the cell, the cytosol. But many of them must then be transported through phospholipid membranes to reach their final destinations, which might be intracellular compartments or even, following secretion, outside the cell(1-5).
2) The molecular mechanism of this "translocation" process has been the subject of elegant biochemical(1,4,5), genetic(3), and biophysical studies. This body of work has shown that the main secretion pathway in all kingdoms of life involves a heterotrimeric protein complex, or "translocase"(4,5), which forms a tightly controlled conduit that allows newly made proteins to pass through membranes before the proteins fold into their functional shape.
3) van den Berg et al (Nature 2004 427:36) have presented the X-ray crystal structure of the translocase (the SecYE complex) from the single-celled archaeon Methanococcus jannaschii, providing an initial view of the atomic architecture of this universally conserved channel. This structure could be considered the latest triumph of genomics, since the choice of the best translocase for structure determination was made on the basis of empirical examination of the expression, purification and crystallization properties of a range of translocases from organisms with fully sequenced genomes.
4) Almost 30 years ago, as a corollary to his "signal hypothesis", Guenter Blobel1, (2) proposed that the translocation of proteins across membranes would occur through a proteinaceous channel of the kind now crystallized. Blobel's research at that time had shown that newly made proteins ("preproteins") that are targeted for export from the cytosol have an extension at one end, called a signal peptide, that is removed during passage through the membrane(1). The hypothesis proposed that different types of extension would function as signals, directing newly made proteins to different membrane-bounded compartments(1,2), and the importance of this insight was rapidly accepted. The proposal that protein translocation occurs through a protein channel remained controversial for some time, until later experiments(3-5) confirmed it.
5) Ion channels have received considerable attention because they show remarkable specificity in allowing one kind of ion through while preventing the passage of very similar molecular species. The protein-translocation channel faces what could be considered a more daunting task, in that it must allow the passage of chemically and sterically varied substrates -- representing any segment from a translocating protein -- without compromising the permeability barrier of the membrane(2,4,5). Blobel's original answer to this conundrum was that translocation would occur at the same time that the protein was being made(1) (co-translationally), with a tight seal between the protein-synthesis machinery (ribosomes) and the pore of the translocation channel maintaining osmotic integrity. But biochemical studies have shown that translocation occurs at least partially post-translationally(4,5), and cryo-electron-microscopic studies show a gap of some 15 angstroms separating the ribosomal exit site from the translocase in detergent-solubilized samples performing co-translational translocation. So the pore of the channel seems likely to have variable but controlled conformational properties, expanding just enough to accommodate larger protein segments without compromising the integrity of the osmotic seal.
References (abridged):
1. Blobel, G. & Dobberstein, B. J. Cell Biol. 67, 835-851 (1975)
2. Blobel, G. Proc. Natl Acad. Sci. USA 77, 1496-1500 (1980)
3. Schatz, P. J. & Beckwith, J. Annu. Rev. Genet. 24, 215-248 (1990)
4. de Keyzer, J., van der Does, C. & Driessen, A. J. Cell. Mol. Life Sci. 60, 2034-2052 (2003)
5. Rapoport, T. A., Jungnickel, B. & Kutay, U. Annu. Rev. Biochem. 65, 271-303 (1996)
Nature http://www.nature.com/nature
--------------------------------
CELL BIOLOGY: PROTEIN SORTING AND GOLGI COMPARTMENTS
The following points are made by B.B. Allen and W.E. Balch (Science 1999 285:63):
1) Movement of cargo between cell compartments requires transiently *coated vesicle carriers. Biosynthetic cargo exiting the endoplasmic reticulum includes the newly synthesized proteins and lipids that are moved to distinct cellular and extracellular destinations. Other cargo incorporated into vesicles includes proteins that are continuously recycled between compartments. These components encompass the transport machinery involved in cargo selection, vesicle formation, and targeting and fusion of vesicles.
2) A fundamental principle of membrane traffic is that vesicle formation is initiated by the selection and concentration of cargo. This occurs through interactions between sorting determinants (markers) on the cargo and cytosolic coat components that direct cargo to the forming vesicle. Soluble cargo (cargo found in the lumen of the ER compartment) will necessarily require sorting receptors to couple the protein to the cytosolic coat machinery. A variety of coat complexes participate in vesicle formation.
3) The authors pose the question: How does the Golgi stack of cisternae mediate transport of cargo from the endoplasmic reticulum to the cell surface? The authors suggest a possibility is that cargo-containing vesicles derived from the endoplasmic reticulum form early Golgi compartments that then mature by retrieval of processing enzymes from later Golgi compartments. Maturation continues at terminal Golgi compartments by retrieval of transport components from the endocytic pathway to promote sorting of cargo to multiple destinations. Thus, the authors suggest, retrograde movement may integrate exocytic (secretory) and endocytic (material uptake) pathways in eukaryotic cells and coordinate membrane flow and cargo transport through the Golgi stack.
Science http://www.sciencemag.org
--------------------------------
Notes:
coated vesicle carriers: Coated vesicles are observed in the cytoplasm of many eukaryotic cells. They measure 50 to 250 nanometers in diameter, and are characterized by a coat made up of a polyhedral lattice of clathrin subunits together with smaller amounts of other proteins. Coated vesicles are concerned with the rapid and continuous transport of molecules between specific membranous organelles of the cell and to and from the cell membrane.
ScienceWeek http://scienceweek.com
--------------------------------
CELL BIOLOGY: MOVEMENTS OF PROTEINS IN MEMBRANES
The following points are made by Katsuyoshi Mihara (Nature 2003 424:505):
1) The "organelles" of plant, animal and yeast cells are specialized compartments that fulfill specific functions. Each organelle is bounded by a lipid membrane, which contains "translocase complexes" that ferry proteins from outside the compartment to the inside. But organelle membranes are more than just barriers; besides the translocase complexes, they contain many other proteins, which often adopt intricate configurations within the membrane itself. The prevailing view is that these proteins are sorted and assembled in the membrane by the same translocase complex that transports proteins across the membrane to the organelle interior. However, recent evidence indicates that in yeast mitochondria additional machinery, besides the translocase complex, is required to sort and assemble mitochondrial outer-membrane proteins that have a complicated conformation -- including the translocase proteins themselves.
2) Mitochondria, the powerhouses of the cell, are bounded by not one, but two membranes. Some mitochondrial proteins reside in one of these membranes, some occur in the space between the membranes, and yet others are at the heart of the mitochondrion. All of these proteins are synthesized as precursor proteins (preproteins) inside the cell and are shuttled across the membranes by TOM and TIM complexes -- preprotein translocases of the outer and inner membranes, respectively. The TOM complex forms a channel in the outer membrane, called the "general insertion pore", through which nearly all mitochondrial preproteins pass. The channel is made by the protein Tom40. This protein chain spans the membrane many times, forming an intricate pore-shaped structure. The entire TOM complex, however, is composed of many different Tom proteins, which together have a relative molecular mass of 400,000.
3) But how are the mature TOM complexes themselves assembled and inserted into the membrane? Some details are already known. Newly synthesized Tom40 precursor protein is recognized by the import receptor Tom20 and is then shuttled across the outer membrane through pre-existing TOM channels. The assembly of new translocases then occurs through two intermediate structures. First, the pore-forming Tom40 preprotein assembles with Tom5 to form a 250K intermediate complex (intermediate I), which is associated with the inner surface of the outer membrane. Second, this complex rearranges into a 100K structure (intermediate II), concomitant with the integration of Tom40 into the membrane. This step is followed by the sequential addition of several other Tom proteins to form the mature TOM complex.
Nature http://www.nature.com/nature
--------------------------------
MITOCHONDRIA: MECHANISMS OF PROTEIN IMPORT
The following points are made by Truscott et al (Current Biology 2003 13:R326):
1) Protein trafficking is a vital cellular process in which proteins are transported from their site of synthesis to locations within cells where they function. In eukaryotes these locations are numerous, as cells are organized into several compartments formed from single or multiple membrane boundaries. Nuclear-encoded proteins, which are synthesized in the cytosol, are transported for example to organelles such as mitochondria, the endoplasmic reticulum (ER), peroxisomes, the nucleus and chloroplasts.
2) Apart from a handful of proteins encoded by the mitochondrial genome, most proteins residing in this organelle are nuclear-encoded and synthesized in the cytosol. Thus, delivery of proteins to their final destination depends on a network of specialized import components that form at least four main translocation complexes. The import machinery ensures that proteins earmarked for the mitochondrion are recognized and delivered to the organelle, transported across membranes, sorted to the correct compartment and assisted in overcoming energetic barriers.
Current Biology http://www.current-biology.com
ScienceWeek http://scienceweek.com
|