Cooperativity myosin-actin interaction

A primary function of the SH3 domains is to form fimctional oligomeric complexes at defined subcellular sites, frequently in cooperation with other modular domains. SH3 domains are foimd in many proteins associated with the cytoskeleton or with the plasma membrane. Examples are the actin binding protein a-spectrin and myosin lb. Furthermore, SH3 interactions are involved in signal transduction in the Ras pathway (see Chapter 9). 

Lehrer, S. S., Golitsina, N. L., and Geeves, M. A. (1997). Actin-tropomyosin activation of myosin subfragment 1 ATPase and thin filament cooperativity. The role of tropomyosin flexibility and end-to-end interactions. Biochemistry 36, 13449-13454. 

The two heads of smooth muscle myosin interact cooperatively. Studies with the soluble two-headed HMM fragment clearly demonstrate that, whereas the phosphorylation of the two heads by myosin light chain kinase was random, phosphorylation of both heads was required for the MgATPase activity of either head to be activated by actin (Sellers et al., 1983 Per-sechini and Hartshorne, 1981). This cooperative activation of the two heads, coupled with the random phosphorylation, meant that at 50% phosphorylation (i.e., 1 mol Pj/mol HMM) there was only 25% of the... 

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