Fibrous Proteins

Although the fibrous proteins represent a diverse collection of structures, they can nonetheless be grouped conveniently into four classes: (i) the a-fibrous proteins, (ii) the b-fibrous structures, (iii) the collagen proteins, and (iv) those proteins that assemble into filamentous arrays but are individually globular in form. In general, all fibrous proteins contain strong repeating elements in their structures, often in the form of tandem sequence motifs. These motifs not only specify the secondary structure, but also play a recognizable and identifiable role in the assembly to higher levels of order. Fibrous proteins have often been used as model systems to provide insights into the structures of the more complex globular proteins.

 1. a-Fibrous Structures

a-Fibrous proteins can exist in vivo either as individual elongated molecules (such as laminin in basement membranes and fibrinogen in blood plasma) or as filamentous assemblies (such as myosin in muscle thick filaments, desmin in Type III intermediate filaments, and fibrin molecules in blood clots). The protein sequences of all members of this family have a high propensity to form a-helices, which are stabilized by hydrogen bonds that lie approximately parallel to the helix axis. In addition, the a-fibrous proteins contain a characteristic heptad repeat  in which nonpolar residues alternate three and four residues apart. The apolar stripe on the surface of the right-handed a-helix becomes internalized when several such a-helices aggregate and assemble into a left-handed ropelike structure (1). Although hydrophobic forces are very important in driving the assembly, interchain electrostatic interactions contribute significantly to determining the relative chain orientations and axial stagger, as well as the stability (1-3). In these filament-forming structures, the amino acid sequence contains a highly regular linear disposition of acidic and basic residues. The periods are usually 180° out of phase, which generates a simple rod structure with alternating bands of positive and negative charge. Consequently, assembly into the filamentous form is specified in large part by maximization of intermolecular ionic interactions, rather akin to an ionic zipper. Those a-fibrous proteins lacking periodicities in charged residues do not form regular filamentous assemblies, as judged by data currently available. In these cases, the molecules exist as separate entities, or assembly occurs in a less regular manner via non-helical domains elsewhere in the molecule, most typically at the N- and C-terminal ends.

 2. b-Fibrous Structures

 The b-fibrous structures were first observed in silk proteins. The fundamental structure is that of an extended array of b- strands held together in a b-sheet by a regular disposition of hydrogen bonds perpendicular to the axes of the chains. Two or more such sheets then aggregate into a b-crystallite. Relatively few sequence data on silks are available, but it is likely that many of them have repeating amino acid sequence motifs. This is based on the observation that the amino acid compositions are generally very simple, with perhaps only a small number of amino acids, such as glycine, alanine, serine, and glutamine, represented in any significant number. On this basis it is thought that many silks contain a dipeptide repeat (or possibly a multiple), although there are non-b-forming examples, such as Nematus ribesii and Apis mellifera, that almost certainly have three- and seven-residue repeats, because they give rise to a collagen and a-fibrous X-ray fiber diffraction pattern, respectively. Those silks with a dipeptide repeat, however, automatically give rise to a b-sheet with the two residues in the repeat spatially separated on opposite sides. This can give a sheet with an apolar face, for example, which can then readily assemble with the same face in a second sheet. It has been assumed that the b-sheets are planar in the silk structures, but this may not be the case in vivo. In fact, it is known that the b-sheets are twisted, probably in a right-handed manner, for both feather and scale keratins. In all fibrous proteins, the b-sheets are composed of antiparallel chains: in globular proteins both parallel and antiparallel b-sheets are found.

3.  Collagen Class

The collagen class of fibrous proteins is characterized by a triplet repeat of the form (Gly–X–Y) n, where X and Y are often proline and hydroxyproline. Type I collagen molecules, each of length 300 nm, aggregate with an axial stagger of distance D (67 nm) or a multiple to generate fibrils. Collagen types II, III, V, and XI also form D-periodic fibrils, but type VI collagen forms fibrils with a different periodicity. Other collagen types (IV, VII, VIII, IX, X, XII, XIII, and XIV( are non-fibril-forming but can be classified further as either basement membrane collagens (IV, VII,(  short-chain collagens (VIII, X), or fibril-associated collagens (IX, XII, XIV).

4. Filamentous Arrays

The last group of fibrous proteins comprise proteins that are globular in shape but assemble either helically or as a long string to form, respectively, a regular or semiregular filamentous structure. A good example of the former system is actin, in which G-actin (globular) assembles into F-actin filaments (fibrous). These, together with other proteins such as tropomyosin and troponin, form the thin filaments of muscle. Titin, a protein from muscle, consists of a series of globular domains strung together and is thus an example of the second type of filament-forming fibrous protein. The short arms in laminin are likewise composed of globular regions strung together in a near linear manner.

References

1. C. Cohen and D. A. D. Parry (1990) -helical coiled-coils and bundles: how to design an -helical bundle. Proteins Struct. Funct. Genet. 7, 1–15. 

2. D. Krylov, I. Mikhailenko, and C. Vinson (1994) A thermodynamic scale for leucine zipper stability and dimerization specificity: e and g interhelical interactions. EMBO J. 13, 2849–2861. 

3. O. D. Monera, C. M. Kay, and R. S. Hodges (1994) Electrostatic interactions control parallel and antiparallel orientation of -helical chains in two-stranded -helical coiled-coils. Biochemistry 33, 3862-3871.

4. D. J. S. Hulmes, A. Miller, D. A. D. Parry, K. A. Piez, and J. Woodhead-Galloway (1973( Analysis of the primary structure of collagen for the origins of molecular packing. J. Mol. Biol. 79, 137-148.