Mutagenesis of a Tropomyosin Isoform from Atlantic Salmon

Atlantic salmon striated muscle tropomyosins unfold over a lower temperature range than their mammalian counterparts. A single sarcomeric isoform belonging to the alpha-class exists in the fast skeletal muscle of the trunk. It shares 20 amino acids substitutions with rabbit alpha-skeletal tropomyosin but migrates closer to the beta isoform (40 substitutions) when electrophoresed in the presence of SDS. Interesting aspects of the sequence heterogeneity (versus rabbit alpha) include: a change in charge at neutrality - salmon fast skeletal tropomyosin is of greater net negative charge - the content of core residues in the carboxyl-terminal half of the molecule and glycine in the amino-terminal half. Mutation of threonine-77 in salmon to the corresponding residue in rabbit, lysine, results in an electrophoretic shift in the absence as well as the presence of SDS. The contribution of the various isomorphisms to the properties of salmon tropomyosin is under investigation using circular dichroism and limited proteolysis.