Intermediate filaments helical proteins

Figure 14.5 The domain organization of intermediate filament protein monomers. Most intermediate filament proteins share a similar rod domain that is usually about 310 amino acids long and forms an extended a helix. The amino-terminal and carboxy-terminal domains are non-a-helical and vary greatly in size and sequence in different intermediate filaments.

An individual polypeptide in the a-keratin coiled coil has a relatively simple tertiary structure, dominated by an a-helical secondary structure with its helical axis twisted in a left-handed superhelix. The intertwining of the two a-helical polypeptides is an example of quaternary structure. Coiled coils of this type are common structural elements in filamentous proteins and in the muscle protein myosin (see Fig. 5-29). The quaternary structure of a-keratin can be quite complex. Many coiled coils can be assembled into large supramolecular complexes, such as the arrangement of a-keratin to form the intermediate filament of hair (Fig. 4-1 lb). 

The two-stranded a-helical coiled coil is now recognized as one of natures favorite ways of creating a dimerization motif and has been predicted to occur in a diverse group of over 200 proteins.111 This structure consists of two amphipathic, right-handed a-helices that adopt a left-handed supercoil, analogous to a two-stranded rope where the nonpolar face of each a-helix is continually adjacent to that of the other helix. 2 This structure was first postulated by Crick to explain the X-ray diffraction pattern of a-keratin in the absence of sequence information.Pl The coiled-coil dimerization motif is natures way of creating a rod-like molecule that perhaps plays only a structural role in many fibrous proteins, such as the kmef (keratin, myosin, epidermis, fibrinogen) class 3,4 and the intermediate filament proteins)5 6 ... 

Smith, T. A., Hempstead, P. D., Palliser, C. C., and Parry, D. A. D. (2003). Modeling o-helical coiled-coil interactions The axial and azimuthal alignment of IB segments from vimentin intermediate filaments. Proteins 50, 207-212. 

Together with actin microfilaments and microtubules, keratin filaments make up the cytoskeleton of vertebrate epithelial cells. Keratins belong to a family of intermediate filament proteins that form a-helical coiled-coil dimers that associate laterally and end to end to form 10 nm diameter filaments. Keratin and actin filaments and microtubules form an integrated cytoskeleton that preserves the shape and structural integrity of the ker-atinocyte as well as serves to transmit mechanical loads. Keratins account for about 30% of the total protein in basal cells. 

Cytoskeleton is defined as the sum of the various filamentous proteins of eukaryotic cells that remain after the cells are extracted with a mild detergent. The cytoskeleton includes actin filaments, two-stranded helical polymers, which form the microfilaments and the actin-binding proteins. Other components are microtubules and intermediate filaments. The cytoskeleton has not only a role in maintaining the shape of cells, it is also actively engaged in cell division, in the organisation and the dynamic movement of ceD organelles and in the movement of cells in chemotaxis. 

In most cells the intermediate filaments provide the scaffolding for the cytoskeleton They may account for only 1% of the protein in a cell but provide up to 85% of the protein in the tough outer layers of skin. Intermediate filament proteins are encoded by over 50 human genes which specify proteins of various sizes, structures, and properties. However, all of them have central 300- to 330-residue a-helical regions through which the molecules associate in parallel pairs to form coiled-coil rods with globular domains at the ends (Fig. 7-31). Some of these proteins, such as the keratin of skin, are insoluble. Others, including the nuclear lamins (Chapter 27) and vimentin, dissociate and reform filaments reversibly. 

C -Keratin, which is the primary component of wool and hair, consists of two right-handed o helices intertwined to form a type of left-handed superhelix called an a coiled coil, ot-Keratin is a member of a superfamily of proteins referred to as coiled-coil proteins (Figure 2,43). In these proteins, two or more a helices can entwine to form a verv stable structure, which can have a length of 1000 A (100 nm, or 0.1 jiim) or more. There are approximately 60 members of this family in humans, including intermediate filaments, proteins that contribute to the cell cytoskeleton (internal scaffolding in a cell), and the muscle proteins myosin and tropomyosin (Section 34.2). Members of this family are characterized by a central region of 300 amino acids that contains imperfect repeats ol a sequence of seven amino acids called a heptad repeal. 

Each intermediate filament protein has globular head and tail domains joined by a long a-helical domain. The a-helical domain contains many direct repeats of an amino-acid sequence motif called the heptad repeat. Since an a-helix makes just under two turns in seven residues, each position in a seven-residue repeat will slowly wind around the helix. Thus, if two such helices with heptad repeats interact through hydrophobic interactions of side chains, they will slowly wind around each other. This structure is known as an a-helical coiled coil and is stabilized by the presence of hydrophobic residues at positions 1 and 4 of the repeat. The intermediate filament is built from pairs of proteins in the coiled-coil configuration. Two of these pairs bind together, in an antiparallel, staggered, configuration. Further pairs can then add on where the overlap occurs (Fig. 10.4). 

Keratins - ot-Keratins are the major proteins of hair and fingernails and a compose a major fraction of animal skin, oi-keratins are classified in the broad group of intermediate filament proteins, which play important structural roles in nuclei, cytoplasm, and cell surfaces. Their secondary structure is composed predominantly of -helices. Figure 6.11 shows the coiled-coil structure of the ot-keratin in hair. The chemical composition of the cysteine residues in ot-keratin affects its macromolecular structure and function. For example, hair has relatively few cysteine cross-links, whereas fingernails have many such cross-links, / -keratins, on the other hand, contain much more pleated sheet secondary strucure than ot-keratins and are found in feathers and scales. 

Keratins, a group of fibrous proteins occurring in wool, hair, hooves, claws, horns and feathers, and also in skin, connective tissues and intermediate filaments. a-Keratins occur in mammals, having around 30 various variants, whereas j3-keratins are found in birds, reptiles, and in the silk of insects and arachnids. a-Keratin forms closely associated pairs of a-helices in which each pair is composed of a type I and a type II keratin chain twisted in parallel into a... 

These dimers aggregate in an antiparallel arrangement to form structural units composed of four protein chains or tetramers [101,102], Seven to ten of these tetramer units are then believed to combine or aggregate into a larger helical structure, forming the intermediate filaments (the microfibril structures) of animal hairs. 

The subunits that constitute the intermediate filaments of hair fibers are polypeptide chains of proteins see Figure 1-33. The coiled sections or the helical domains of these protein chains are approximately 10 angstroms in diameter, including side chains, and are believed to approximate the form of an alpha helix, first proposed by Pauling and Corey [103-105] (see Figures 1-34 and 1-35). 

The organizational level believed to control the swelling behavior of keratins is the secondary and tertiary structure of the intermediate filaments and the matrix [116, 117]. As indicated previously, the intermediate filaments consist of proteins containing alpha-helical segments embedded in the less organized matrix of high cystine content. 

In keratin fibers like human hair and wool fiber, the helical proteins of the intermediate filaments (microfibrils) are oriented parallel to the axis of... 

The intermediate filaments in different tissues appear similar in form (see Chapter 1), but they differ considerably in their exact composition. The common structural feature among this class of proteins is the central helical rod [111]. On the other hand, the primary differences are in the amino and carboxyl domains, and these vary in both amino acid sequences and size... 

Fig. 3 Schematic view of the human vimentin protein and force-strain curves of coiled-coil intermediate filament under tensile loadings, (a) Schematic representation of vimentin structure, (b) Force-strain behaviors of a coiled-coil a-helical structures revealing the loading rate dependency of the molecular-level stiffness under tensile loading. (Reprinted from [66], with kind permission from Springer Science and Business Media), (c) a-p secondary structural transition of coiled-coil a-helix under tensile loading. (Reprinted from [67])...

Wool and hair have the most complex structures of any textile fibres. In the paper by Viney, fig. 1 shows how keratin proteins, of which there are more than one type, all having a complicated sequence of amino acids, assemble into intermediate filaments (IFs or microfibrils). But, as shown in Fig. 5a, this is only one part of the story. The microfibrils are embedded in a matrix, as shown in Fig. 5b. The keratin-associated proteins of the matrix contain substantial amounts of cy.stine, which cross-links molecules by -CH2-S-S-CH2- groups. Furthermore, terminal domains (tails) of the IFs, which also contain cystine, project into the matrix and join the cross-linked network. At a coarser scale, as indicated in Fig. 5c, wool is composed of cells, which are bonded together by the cell membrane complex (CMC), which is rich in lipids. As a whole, wool has a multi-component form, which consists of para-cortex, ortho-cortex, meso-cortex (not shown in Fig. 5a), and a multi-layer cuticle. In the para-and meso-cortex the fibril-matrix is a parallel assembly and the macrofibrils, if they are present, run into one another, but in the ortho-cortex the fibrils are assembled as helically twisted macrofibrils, which are clearly apparent in cross-section.s. 

Figure 2 Composite structure of hair at various length-scales (a) filament protein with alternating helical/linker sections (helical section is shown atttie bottom) (b) coiled coil of filament proteins (in a polar environment two polypeptide chains naturally coil around each other in parallel, as this leaves the hydrophobic groups shielded in the centre) (c) intermediate filament with 16 coils (d) filament embedded in matrix (e) macrofibril (f) cortical call enclosed by cell-membrane complex.

Intermediate filament proteins are classified into subtypes I-VI of which Types I and II are, respectively, acidic and basic keratins [52]. lype III contains vi-mentin, the most widely distributed intermediate filament protein [53]. Desmin is another type of intermediate filament [54]. All these proteins have a-helical conformations with terminal non-helical globular domains and they are all destined to form coiled coil secondary structures. The structural and functional studies of keratin and keratin-like proteins are actively pursued in various animal models [55, 56]. 

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