The I130V and S229F Tropomyosin Mutations Associated With Congenital Heart Disease Disrupt Actin-Myosin Interactions in the Atria.

Abstract

Tropomyosin is a highly conserved α-helical coiled-coil protein that is involved in maintaining the structural integrity of the sarcomere and regulating contraction of striated muscle. Mutations in the TPM1 gene encoding cardiac α-tropomyosin (Tpm1.1) are associated with left ventricular noncompact cardiomyopathy and congenital heart defects (CHD). We investigated the structural and functional properties of Tpm1.1 with the I130V and S229F mutations associated with CHD. We found that both mutations decreased the Tpm affinity for F-actin and differently affected the sliding velocity of F-actin-Tpm filaments over ventricular and atrial myosin in an in vitro motility assay. The results indicate a disruption in the activation of F-actin-Tpm filaments by myosin cross-bridges with both atrial and ventricular myosin. The I130V mutation increased the maximum sliding velocity of thin filaments over atrial myosin and decreased the Ca²⁺ sensitivity of the filament velocity. The S229F mutation increased the maximum filament velocity on atrial myosin and decreased the Hill coefficient. The mutations did not affect the Ca²⁺-dependent characteristics of thin filament velocity with ventricular myosin. Thus, the molecular mechanism of CHD pathogenesis induced by Tpm1.1 mutations is associated with changes in sarcomere stability and disruption of actin-myosin interaction in the myocardium of the ventricles and atria, which negatively affect the contractility of both heart chambers.