Tropomyosin surface residues

Repeating surface residues of tropomyosin appear to interact direcdy with recognition sites on consecutive monomers of F-actin. We first discuss, however, the repeat in the core, which, although less regular than that of certain surface residues (see below), has a clearly understood effect on the conformational and dynamic properties of the coiled-coil. 

The seven distinct amino acid positions and associated interactions that are produced from the a-helical coiled-coil provide the basic structural unit of tropomyosin. These elements superimpose on the longer, roughly 40-residue, functional unit of tropomyosin (see Fig. 1), and patterns of residues found both in the core of the coiled-coil as well as on its surface are repeated seven times in a full-length tropomyosin molecule and play a role in the periodic binding of tropomyosin to actin. 

Analyses of the tropomyosin sequence have shown long-range periodicities of certain surface acidic and apolar residues that are likely to be recognition sites for actin. These features are discussed below in relation to their role in regulation (see Turning on the Thin Filament section). 

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