Effects of two familial hypertrophic cardiomyopathy mutations in alpha-tropomyosin, Asp175Asn and Glu180Gly, on the thermal unfolding of actin-bound tropomyosin

Differential scanning calorimetry was used to investigate the thermal unfolding of native alpha-tropomyosin (Tm), wild-type alpha-Tm expressed in Escherichia coli and the wild-type alpha-Tm carrying either of two missense mutations associated with familial hypertrophic cardiomyopathy, D175N or E180G. Recombinant alpha-Tm was expressed with an N-terminal Ala-Ser extension to substitute for the essential N-terminal acetylation of the native Tm. Native and Ala-Ser-Tm were indistinguishable in our assays. In the absence of F-actin, the thermal unfolding of Tm was reversible and the heat sorption curve of Tm with Cys-190 reduced was decomposed into two separate calorimetric domains with maxima at approximately 42 and 51 degrees C. In the presence of phalloidin-stabilized F-actin, a new cooperative transition appears at 46-47 degrees C and completely disappears after the irreversible denaturation of F-actin. A good correlation was found to exist between the maximum of this peak and the temperature of half-maximal dissociation of the F-actin/Tm complex as determined by light scattering experiments. We conclude that Tm thermal denaturation only occurs upon its dissociation from F-actin. In the presence of F-actin, D175N alpha-Tm shows a melting profile and temperature dependence of dissociation from F-actin similar to those for wild-type alpha-Tm. The actin-induced stabilization of E180G alpha-Tm is significantly less than for wild-type alpha-Tm and D175N alpha-Tm, and this property could contribute to the more severe myopathy phenotype reported for this mutation.