THE BINDING OF DISTINCT SEGMENTS OF ACTIN TO MULTIPLE SITES IN THE C-TERMINUS OF CALDESMON - COMPARATIVE ASPECTS OF ACTIN INTERACTION WITH TROPONIN-I AND CALDESMON

Thin-filament-based regulation of the contractile response is consider ed to involve the interaction of actin with troponin-I in striated mus cle and the interaction of actin with caldesmon in smooth muscle. The nature of the interaction with actin of these inhibitory proteins has been studied by proton magnetic resonance spectroscopy using segments of caldesmon and troponin-I which mimic their functional properties. C aldesmon is shown to interact with two distinct sites on the N-termina l residues 1-44 of actin subdomain I with corresponding contacts on ca ldesmon domain 3 and domain 4 at its C-terminus. We demonstrate that, whereas inhibition by the troponin-I fragment (residues 96-117) is eff ected by its interaction with the N-terminal region of actin, the sepa rate inhibitory ability of different regions of the C-terminus of cald esmon (domains 4a and 4b) is mediated by interaction with noncontiguou s segments on subdomain 1 of actin. Our studies of the spatial relatio nship of these actin contacts on caldesmon further suggest that one mo lecule of caldesmon may associate with two actin monomers. The demonst rated interactive nature of these caldesmon attachments to distinct re gions of actin is relevant to the mechanism of calcium modulation of i nhibition of actomyosin ATPase by caldesmon.