Conformational properties of striated muscle tropomyosins from some salmonid fishes.

The conformational stability of tropomyosins from salmonids fishes has been investigated under a variety of conditions (salt, pH and osmolyte) using electronic circular dichroism. Every salmonid tropomyosin (from: fast skeletal muscle; slow skeletal muscle and heart) is less resistant to heat-induced denaturation than rabbit alpha-striated tropomyosin. Induction of unfolding, by application of a linear temperature gradient, yields a distinct profile for each protein (0.1 M salt, pH 7, plus dithiothreitol): fast tropomyosin (Tms 24 and 40 degrees C major); cardiac tropomyosin (Tm, 36 degrees C) and slow tropomyosin (Tms, 39 major and 47 degrees C). Correlation of these results, and others obtained under different solvent conditions, with the known sequences (Jackman DM, Waddleton DM, Younghusband B, Heeley DH (1996) Further characterisation of fast, slow and cardiac muscle tropomyosins from salmonid fish. Eur. J. Biochem. 242:363-371) provides insight into how the coiled-coil may have adapted to cold. The most variable sections of sequence encompass residues 9-49, 73-87 and 172-216. In two of these regions there is a pair of closely-spaced glycines, namely at residues 24 and 27 in fast skeletal tropomyosin and residues 83 and 87 in cardiac tropomyosin. A region of low stability is located in the carboxy-terminal half of the isoform from fast skeletal muscle. This segment cooperatively unfolds in the 20 degrees range and accounts for 20% of the total far-UV ellipticity change under reducing conditions. It is unresponsive to increasing ionic strength and the presence of osmolyte but is sensitive to oxidation at cysteine 190. A likely contributor to the relative instability is a substitution at position 179 whereby fast skeletal tropomyosin, but not the other tropomyosins under examination, has lost a "d" position alanine in the fifth cluster and gained a polar side-chain.