Tropomyosins regulate the impact of actin binding proteins on actin filaments.

The state of actin depends intimately on its interaction partners in eukaryotic cells. Classically, the cooperative force-generating acto-myosin couple is turned off and on by the calcium-dependent binding and release of tropomyosin molecules. The situation with nonmuscle cells appears to be much more complicated, with tropomyosin isoforms regulating the kinds of tension-producing and stress-bearing structures formed of actin filaments. The polymerization of even the shortest gelsolin-capped filaments is efficiently promoted by the binding of tropomyosin, for example, a process that might occur all the way out to the leading edges of advancing cells. Recently, multimers of tropomyosin have been discovered that appear to be assembly intermediates, formed from identical tropomyosin molecules, which act as ready pools of tropomyosin during the catalytic formation of lamellipodia and filopodia. Remarkably, these multimers apparently reform during the disassembly of cellular actin-containing structures. The existence of these recyclable, tropomyosin isoform-specific structures suggests how cells prevent nonproductive association of non-identical, but closely similar, tropomyosin isoforms.