EFFECTS OF 2 FAMILIAL HYPERTROPHIC CARDIOMYOPATHY-CAUSING MUTATIONS ON ALPHA-TROPOMYOSIN STRUCTURE AND FUNCTION

Missense mutations in cl-tropomyosin can cause familial hypertrophic c ardiomyopathy. The effects of two of these, Asp175Asn and Glu180Gly, h ave been tested on the structure and function of recombinant human tro pomyosin expressed in Escherichia coli. The F-actin affinity (measured by cosedimentation) of Glu180Gly was similar to that of wild-type, bu t Asp175Asn was more than 2-fold weaker, whether or not troponin was p resent. The mutations had no apparent effect on the affinity of tropom yosin for troponin. The mutations had a small effect on the overall st ability (measured using circular dichroism) but caused increased local flexibility or decreased local stability, as evaluated by the higher excimer/monomer ratios of tropomyosin labeled with pyrene maleimide at Cys 190. The pyrene-labeled tropomyosins differed in their response t o myosin S1 binding to the actin-tropomyosin filament. The conformatio ns of the two mutants were different from each other and from wild-typ e in the myosin S1-induced on-state of the thin filament. Even though both mutant tropomyosins bound cooperatively to actin, they did not re spond with the same conformational change as wild-type when myosin S1 switched the thin filament from the off- to the on-state.