Dimerization of tropomyosins.

Tropomyosins consist of nearly 100% alpha-helix and assemble into paralleldimeric coiled-coils. Nonmusde as well as muscle tropomyosins can form homodimers, however, expression of both muscle alpha and beta tropomyosin subunits results in the preferential formation of stable alpha/beta heterodimers in native muscle. The assembly preference of the muscle tropomyosin heterodimer can be understood in terms of its thermodynamically favorable energy distribution that provides increased stability over the homodimer. The simultaneous expression of multiple tropomyosin isoforms in nonmuscle cells (at least up to seven individual chains), however, points towards a more complex principle for determining dimer preference. The information for homo- and heterodimerization is contained within the tropomyosin molecule itself and the parameters for dimer selectivity are conferred in part by the alternatively spliced exons. However, it remains to be established if low molecular weight tropomyosin isoforms in nonmuscle cells engage in both homdimer and heterodimer formation in vivo. A thorough understanding of the selective dimer formation of the more than 40 tropomyosin isoforms is required to explain how subtle alterations in the sequence of one tropomyosin chain can result in the progression of diverse disease phenotypes.