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ScienceWeek
CELL BIOLOGY: ON INTERMEDIATE FILAMENT PROTEINS
The following points are made by M.B. Omary et al (New Engl. J. Med. 2004 351:2087):
1) The cell cytoskeleton consists of three abundant families of fibrillary proteins: microfilaments, microtubules, and intermediate filaments.(1,2) Intermediate filament proteins derive their name from their diameter, which is intermediate between the diameters of microfilaments and microtubules.(1,2) They differ from actin microfilaments and tubulin microtubules in their large number, their distribution in the cytoplasm and nucleus, their diverse primary structure, their nonpolar architecture, their relative insolubility, and their nucleotide-independent dynamics.(1,2,3)
2) The human genome contains at least 65 functional genes encoding intermediate filament proteins, placing them among the 100 largest gene families in humans.(4) More than 30 diseases are related to mutations in these genes. The majority of them are rare (affecting fewer than 200,000 patients in the US) or difficult to treat, but collectively they affect most tissues, and not all of them are rare, as exemplified by the association of keratin mutations with end-stage liver disease. Intermediate filament proteins have long been considered unique to multicellular eukaryotic organisms, in contrast to microfilaments and microtubules, which have prokaryotic ancestors. However, crescentin, which is found in several curved bacteria, including Caulobacter crescentus and Helicobacter pylori, was recently identified as an intermediate filament-like ancestor protein that accounts for the morphologic features of caulobacter.
3) All intermediate filament proteins have a prototypical structure consisting of a coiled-coil, alpha-helix rod domain (two polypeptide alpha-helixes wound around each other) that is interrupted by linkers and flanked by N-terminal head and C-terminal tail domains.(2,3) The simplest soluble unit of intermediate filament proteins is a tetramer consisting of two antiparallel dimers; each dimer, in the case of keratins, consists of one type I keratin molecule and one type II keratin molecule. A high-resolution architectural model of intermediate filaments is lacking, although the mature, 10-to-12-nm fiber is believed to contain 32 monomers in diameter, with important structural polymorphisms among members of the protein family.(2,3) Cytoplasmic intermediate filaments can assume various network configurations depending on the cell type and manifest variable differentiation-related patterns, such as those in apical poles (e.g., pancreatic keratins), Z lines (desmin), and axonal processes (neurofilaments).(2,3)
4) Intermediate filament proteins are regulated by several post-translational modifications, including farnesylation, phosphorylation, glycosylation, and transglutamination, and by an accumulating number of associated proteins. These modifications and protein associations contribute in key ways to the function and dynamics of intermediate filaments. For example, phosphorylation regulates the filaments' organization and solubility, association with interacting proteins, and susceptibility to degradation during apoptosis. Alterations in intermediate filament gene expression and protein phosphorylation also serve as markers of tissue injury.
5) In addition, transglutamination of intermediate filaments and their associated proteins under physiologic conditions is essential for the formation of the protective, cornified cell envelope that contributes to the structure of the skin barrier. In pathologic conditions, intermediate filament transglutamination and other alterations cause the formation of a variety of intermediate filament-containing inclusion bodies. Intermediate filament proteins are also cleaved by caspases during apoptosis at a conserved motif, and mutations that alter the degradation of keratins during apoptosis have been identified within this motif.(4,5)
References (abridged):
1. Ku N-O, Zhou X, Toivola DM, Omary MB. The cytoskeleton of digestive epithelia in health and disease. Am J Physiol 1999;277:G1108-G1137
2. Fuchs E, Cleveland DW. A structural scaffolding of intermediate filaments in health and disease. Science 1998;279:514-519
3. Herrmann H, Hesse M, Reichenzeller M, Aebi U, Magin TM. Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation. Int Rev Cytol 2003;223:83-175
4. Hesse M, Magin TM, Weber K. Genes for intermediate filament proteins and the draft sequence of the human genome: novel keratin genes and a surprisingly high number of pseudogenes related to keratin genes 8 and 18. J Cell Sci 2001;114:2569-2575
5. Csoka AB, Cao H, Sammak PJ, Constantinescu D, Schatten GP, Hegele RA. Novel lamin A/C gene (LMNA) mutations in atypical progeroid syndromes. J Med Genet 2004;41:304-308
New Engl. J. Med. http://www.nejm.org
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Related Material:
ON INTERMEDIATE FILAMENTS
Notes by ScienceWeek:
The advent of electron microscopy in the 1950s, with practical resolutions at that time of the order of 10 or 20 angstroms, completely changed our view of the interior of biological cells, and for the first time networks of interconnected filaments in protoplasm were recognized as a cytoplasmic framework inside all cells. This framework, called the "cytoskeleton", was demonstrated to be involved in at least two basic functions: a) the provision of a scaffolding supporting and organizing the cell interior, the scaffolding a structured framework that permits cells to assume elaborate shapes, and which organizes and guides interactions among intracellular organelles; and b) The generation of movement, the cytoskeleton containing elements that permit both movement of the cell as a whole and movement of various intracellular components.
The intracellular cytoskeleton is apparently constructed from 3 classes of protein filaments: actin filaments (microfilaments), microtubules, and intermediate filaments. In some cases, a single type of filament is responsible for a particular function; in other cases, interactions between different filament types are involved.
The following points are made by Robert D. Goldman (Proc. Nat. Acad. Sci. 2001 98:7659):
1) Intermediate filaments represent one of the three major cytoskeletal systems found in animal cells, their name derived from their 10-nanometer diameter, which lies between that of smaller actin-containing microfilaments and larger microtubules. The name, however, belies their importance as critical players in the organization of cells and tissues of vertebrate systems. Concerning function, it has become apparent from studies of numerous human disorders, such as those that cause blistering diseases of the skin, that intermediate filaments play important roles in establishing and maintaining the mechanical integrity of cells.
2) Depending on the cell type, intermediate filament proteins comprise anywhere from 1 to 85 percent of total cell protein, but despite these quantities, intermediate filaments remain the least studied and least understood of all cytoskeletal systems. Historically, there are many reasons for this lack of understanding of their structure and function, the most obvious reason relating to the fact that the structural proteins that assemble into intermediate filaments are not highly conserved.
3) For example, humans contain intermediate filaments that are encoded by over 50 different members of a multi-gene family, with this family subdivided into 6 types on the basis of similarities in their amino acid sequences. This is in stark contrast to the other cytoskeletal components, whose core structures are comprised primarily of the highly conserved subunits of microtubules, alpha- and beta-tubulin, and actin, the major subunit of microfilaments.
Proc. Nat. Acad. Sci. http://www.pnas.org
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Related Material:
ON THE REDUCTIONIST APPROACH TO THE CYTOSKELETON
The following points are made by Thomas D. Pollard (Nature 2003 422:741):
1) Our understanding of the cytoskeleton and cellular motility is a triumph of the reductionist strategy, the approach that now dominates research in cell biology. Sophisticated methods drive rapid progress, but we should be aware of the limitations of these methods and the unfulfilled items on the reductionist agenda. The reductionist tasks include an inventory of the relevant molecules, determination of molecular structures, identification of molecular partners, measurement of rate and equilibrium constants for each reaction, localization of the molecules in live cells, physiological tests for participation in cellular processes and formulation of mathematical models to understand the system's behavior.
2) Reductionism starts with a list of the components. Most of the cytoskeletal proteins were discovered the "old-fashioned" way, using purification by biochemical fractionation. Complete genome sequences and expressed sequence tag collections have expanded the inventory of cytoskeletal and motor proteins, particularly the diversity of isoforms of many of the proteins found in higher organisms. In a few cases experts have completed the annotation of selected genomes and defined the size of certain gene families such as myosins, which consists of more than 40 genes in humans. Similar work remains to be done for many other cytoskeletal gene families. Far less is known about the diversity of products generated by alternative splicing of pre-messenger RNAs.
3) Genetic screens and yeast two-hybrid assays have accelerated detection of protein partners, but traditional biochemical assays and affinity chromatography remain useful, particularly when empowered by sensitive analytical methods such as mass spectrometry. When scaled up to sample entire genomes or proteomes, these assays produce impressive interaction maps. Such efforts have saved an immense amount of work and laid out a broad research agenda that is required to understand each interaction. These maps are, of course, a beginning rather than an end, as simple knowledge of an interaction will not explain how anything actually works. Structure determines function, so the field eagerly awaits each new structure.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
The term "cytoskeleton" refers to the quasi-rigid matrix that among other things determines cell shape.
"Motor proteins" are mechanico-chemical enzymes involved in locomotion or transport, and there are three families of such proteins: kinesins, dyneins, and myosins. Kinesins and dyneins are microtubule based motor proteins, while myosin is a microfilament based motor protein. In general, as mechanico-chemical enzymes, motor proteins convert energy from hydrolysis of nucleotides to mechanical force, and since they are involved in many important cellular events, the molecular details are currently the focus of intensive research.
Messenger RNAs (mRNAs) are the ribonucleic acid molecules transcribed from DNA that carry the coded information specifying the sequence of amino acids in proteins.
The "proteome" is the full complement of proteins encoded by the genome.
ScienceWeek http://scienceweek.com
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