Myosin coiled-coil

Chew, M. W., and Squire, J. M. (1995). Packing of alpha-helical coiled-coil myosin rods invertebrate muscle thick filaments./. Struct. Biol. 115, 233-249. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

The leucine zipper DNA-binding proteins, described in Chapter 10, are examples of globular proteins that use coiled coils to form both homo- and heterodimers. A variety of fibrous proteins also have heptad repeats in their sequences and use coiled coils to form oligomers, mainly dimers and trimers. Among these are myosin, fibrinogen, actin cross-linking proteins such as spectrin and dystrophin as well as the intermediate filament proteins keratin, vimentin, desmin, and neurofilament proteins. 

Figure 14.14 Sci ematic diagram of the myosin molecule, comprising two heavy chains (green) that form a coiled-coil tail with two globular heads and four light chains (gray) of two slightly differing sizes, each one bound to each heavy-chain globular head.

Figure 14.15 Stmcture of the SI fragment of chicken myosin as a Richardson diagram (a) and a space-filling model (b). The two light chains are shown in magenta and yellow. The heavy chain is colored according to three proteolytic fragments produced by trypsin a 25-kDa N-terminal domain (green) a central 50-kDa fragment (red) divided by a cleft into a 50K upper and a 50K lower domain and a 20-kDa C-terminal domain (blue) that links the myosin head to the coiled-coil tail. The 50-kDa and 20-kDa domains both bind actin, while the 25-kDa domain binds ATP. [(b) Courtesy of 1. Rayment.]...

Fibrous proteins are long-chain polymers that are used as structural materials. Most contain specific repetitive amino acid sequences and fall into one of three groups coiled-coil a helices as in keratin and myosin triple helices as in collagen and p sheets as in silk and amyloid fibrils. 

FIGURE 17.17 All axial view of the two-stranded, a-helical coiled coll of a myosin tall. Hydrophobic residues a and d of the seven-resldne repeat sequence align to form a hydrophobic core. Residues b, c, and f face the outer surface of the colled coll and are typically Ionic. 

Repeating Structural Elements Are the Secret of Myosin s Coiled Coils... 

FIGURE 17.18 The packing of myosin molecules in a thick filament. Adjoining molecules are offset by approximately 14 nm, a distance corresponding to 98 residues of the coiled coil. 

It is a large protein that consists of six polypeptide chains, two identical heavy chains (each 220 kDa) and two pairs of light chains (each -20 kDa). The two heavy chains form a long coiled unit known as the tail, which associates laterally with other myosin molecules to form the thick filament. At one end of the chain are the two globular heads which form the crossbridges. Each head has two domains, one on either side. 

In the keratins, large parts of the peptide chain show right-handed a-helical coiling. Two chains each form a left-handed superhelix, as is also seen in myosin (see p. 65). The superhelical keratin dimers join to form tetramers, and these aggregate further to form protofilaments, with a diameter of 3 nm. Finally, eight protofilaments then form an intermediate filament, with a diameter of 10 nm (see p.204). 

Steimle, P.A. Licate, L. Cote, G.P. Egelhoff, T.T. Lamellipodial localization of Dictyostelium myosin heavy chain kinase A is mediated via F-actin binding by the coiled-coil domain. FEBS Lett., 516, 58-62 (2002)... 

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). 

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