Shark skeletal muscle tropomyosin is a phosphoprotein.

Shark skeletal muscle tropomyosin is classified as an alpha-type isoform. The chemical structure is characterised by the absence of cysteine and the presence of a sub-stoichiometric amount of covalently bound phosphate. The protein migrates as a single component on a SDS polyacrylamide gel but is resolved into two components by chromatography and electrophoresis both in the presence of urea at mild alkaline pH. The only detectable difference between these components is the presence of phosphoserine in the tropomyosin form of greater net negative charge. Low ionic strength (pH 7) solutions of phosphorylated shark tropomyosin display significantly higher specific viscosity than unphosphorylated, consistent with the presence of a phosphorylation site within the overlap region, serine 283, as well as conservation of the positively charged amino terminal region. Similar observations were made with tropomyosin prepared from the trunk muscle of Atlantic cod. In a steady-state MgATPase assay, thin filaments (Ca2+) reconstituted with shark phosphorylated tropomyosin activate myosin to a greater extent than those composed of unphosphorylated. The difference is attributable chiefly to a change in Vmax. Skeletal muscle tropomyosin is concluded to be phosphorylated in cartilaginous fishes as well as some teleosts.