Comparison of the fluorescence and conformational properties of smooth and striated tropomyosin.

In contrast to previous conformational studies with rabbit skeletal and cardiac tropomyosins, (i) when the cysteine side chains of chicken gizzard tropomyosin were reacted with 5,5'-dithiobis(2-nitrobenzoate), an interchain disulfide cross-link was not produced, (ii) when they were labeled with pyrenylmaleimide , excimer fluorescence was not observed, and (iii) when they were labeled with didansylcystine , a long-lived fluorescence component did not appreciably contribute to the fluorescence decay over a large temperature range including the major unfolding transition. In addition, the temperature dependence of the ellipticity at 222 nm did not reveal a pretransition prior to the main helix unfolding transition. This indicates that gizzard tropomyosin does not exhibit a localized chain-open state in the region of its cysteine residues, analogous to that seen with cardiac and skeletal tropomyosins, nor in any other region of the molecule. As a consequence, these observations suggest that gizzard tropomyosin is more rigid than striated tropomyosin.