The regulatory effects of tropomyosin and troponin-I on the interaction of myosin loop regions with F-actin.

The N terminus of skeletal myosin light chain 1 and the cardiomyopathy loop of human cardiac myosin have been shown previously to bind to actin in the presence and absence of tropomyosin (Patchell, V. B., Gallon, C. E., Hodgkin, M. A., Fattoum, A., Perry, S. V., and Levine, B. A. (2002) Eur. J. Biochem. 269, 5088-5100). We have extended this work and have shown that segments corresponding to other regions of human cardiac beta-myosin, presumed to be sites of interaction with F-actin (residues 554-584, 622-646, and 633-660), likewise bind independently to actin under similar conditions. The binding to F-actin of a peptide spanning the minimal inhibitory segment of human cardiac troponin I (residues 134-147) resulted in the dissociation from F-actin of all the myosin peptides bound to it either individually or in combination. Troponin C neutralized the effect of the inhibitory peptide on the binding of the myosin peptides to F-actin. We conclude that the binding of the inhibitory region of troponin I to actin, which occurs during relaxation in muscle when the calcium concentration is low, imposes conformational changes that are propagated to different locations on the surface of actin. We suggest that the role of tropomyosin is to facilitate the transmission of structural changes along the F-actin filament so that the monomers within a structural unit are able to interact with myosin.