Rod domains

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.

FIGURE 6.14 (a) Both type I and type II o -keratin molecules have sequences consisting of long, central rod domains with terminal cap domains. The numbers of amino acid residues in each domain are indicated. Asterisks denote domains of variable length. 

The central rod domain of a keratin protein is approximately 312 residues in length. What is the length (in A) of the keratin rod domain If this same peptide segment were a true u-helix, how long would it be If the same segment were a /3-sheet, what would its length be ... 

An intracellular fibrous system exists of filaments with an axial periodicity of 21 nm and a diameter of 8-10 nm that is intermediate between that of microfilaments (6 nm) and microtubules (23 nm). Four classes of intermediate filaments are found, as indicated in Table 49-13. They are all elongated, fibrous molecules, with a central rod domain, an amino terminal head, and a carboxyl terminal tail. They form a structure like a rope, and the mature filaments are composed of tetramers packed together in a helical manner. They are important structural components of cells, and most are relatively stable components of the cytoskeleton, not undergoing rapid assembly and disassembly and not... 

Types I and II are the keratins that are found in various combinations in epithelial cells throughout the body. Keratin IFs must include representative subunits of both type I and type II IF subunits with each tissue having a characteristic combination. In contrast, type III IF subunits typically form homopolymers. They include IFs that are characteristic of less differentiated cells like glial or neuronal precursors, as well as those seen in more specialized cell types like smooth muscle cells and mature astrocytes. These three types of IF not only share sequence homology within their rod domains but their genes... 

Figure 7-31 A model for the structure of keratin microfibrils of intermediate filaments. (A) A coiled-coil dimer, 45-nm in length. The helical segments of the rod domains are interrupted by three linker regions. The conformations of the head and tail domains are unknown but are thought to be flexible. (B) Probable organization of a protofilament, involving staggered antiparallel rows of dimers. From Jeffrey A. Cohlberg297...

Burkhard, P., Kammerer, R. A., Steinmetz, M. O., Bourenkov, G. P., and Aebi, U. (2000a). The coiled-coil trigger site of the rod domain of cortexillin I unveils a distinct network of interhelical and intrahelical salt bridges. Structure 8, 223-230. 

Fig. 1. Schematic diagram of (a) an intermediate filament heterodimer with coiled-coil domains 1A, IB, 2A, and 2B, and noncoiled-coil connecting linkers LI, L12, and L2. A stutter occurs in the heptad substructure at a point close to the center of segment 2B. The N-terminal globular domains (green for Type I and brown for Type II chains) are termed the heads, and the C-terminal domains (red for Type I and orange for Type II chains) are designated the tails. In (b), the heads are shown folded back over the rod domain, where it is believed that this will stabilize segment 1A. In (c), the heads are shown away from the body of the rod domain and in a position where they can interact more easily with other cellular entities. As a consequence, segment 1A may become destabilized and hence unwind to form two separate a-helical strands.

The epidermal keratins have a tail subdomain structure closely parallel to that described for the heads in Section II.A.l. Immediately C-terminal to the rod domain exists a stretch of highly conserved sequence across Type II chains (Steinert et al., 1985). This is termed the H2 subdomain and is 20 residues long. An equivalent conserved region in Type I chains, however, is totally absent. C-terminal to H2 lies a sequence that varies between chains within a particular type, but which is rich in glycine and serine residues. This V2 subdomain, present in both Type I and Type II chains, has a length that lies in the range 0-110 and 25-125 residues in the two chain types, respectively. Human K1 and K10 chains both show... 

Fig. 6. Schematic representation of the tail domains in Type IV intermediate filament chains. The rod domain, indicated by a shaded rectangle, does not show its known substructure. The four chains illustrated here are, from the top downwards, a-intemexin, and the low (NF-L), medium (NF-M), and high molecular weight chains (NF-FI) from neurofilaments. The characters of the segments are indicated by the one letter code. Figure is redrawn from Parry and Steinert (1995) and is based on an original by Shaw.

It is evident that specific residue-related features observed in the rod domain of one particular IF chain type are frequently not observed in the same place (if at all) in the other chain types and, of course, the large differences in head and tail structure and sequence between chain types preclude identical roles from occurring there as well. The highly specialized sequence characteristics thus define unique structural assemblies and functions, while still maintaining a high degree of structural uniformity, particularly in the manner of rod domain assembly. Even in this case, small but important differences in the An, A22, and A12 modes occur. Distinct IF structures have been identified for (1) unoxidized trichocyte keratin and epidermal keratin (2) oxidized trichocyte keratin (3) Type III and IV IF proteins and (4) the nuclear lam ins. 

Aetin binding domain Plakin domain Spectrin repeals xmxa Coiled coil rod domain Plakin repeals... 

N-terminal actin-binding domains and in the spectrin repeats that form the rod domains (Davison and Critchley, 1988). The spectrin repeats are found in distinct multiples in each protein, resulting in a characteristic actin crosslinking distance. a-Actinin contains four repeats, /3-spectrin contains 17, a-spectrin contains 20, and dystrophin contains 24. The sequences of some spectrin repeats of a- and /3-spectrin are similar in many ways to the four repeats present in a-actinin (Dubreuil, 1991). Within the cell, a-actinin and spectrin dimerize, although the spectrins interact further to generate a functional tetramer (Fig. 1). Most notable is that the ends of the native spectrin tetramer involved in the dimerization event show remarkable similarity to the rod domain repeats of a-actinin that also mediate dimer formation. 

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