Tolulope Ige - March 14,2012
Mutagenesis of a Tropomycin Isoform from Atlantic Salmon
Tropomyosin is a rod-shaped protein consisting of two alpha-helices arranged in a coiled-coil configuration. In the muscle thin filament complex, tropomyosin molecules interact with each other end-to-end as well as with the proteins actin and troponin. These interactions are vital to the regulation of muscle contraction. Tropomyosin from the fast skeletal (trunk) muscle of Atlantic salmon (Salmo salar) is an alpha-type isoform. It shares ~93% identity with a counterpart from rabbit alpha-skeletal muscle (20 amino acid substitutions out of a total of 284) but is less thermally stable.
An interesting aspect of the heterogeneity is the replacement of lysine-77 in rabbit to threonine in salmon. In the case of the mammalian protein the lysine is positioned to form ion pairs with aspartate-80 in the next turn of the same helix and also glutamate 82 in the opposing helix of the coiled coil. Thus, at neutrality, salmon tropomyosin loses two charge-charge interactions. The contribution of residue-77 to the properties of salmon tropomyosin has been investigated using site-directed mutagenesis, limited proteolysis, affinity chromatography and circular dichroism.