Ca 2 +-binding properties and regulatory roles of lobster troponin C sites II and IV

Invertebrate troponin C typically contains Ca-specific binding sites, sites II and IV, in the Nand C-terminal domains, respectively. To investigate the roles of these sites for Ca-dependent regulation of muscle contraction, we generated lobster troponin C mutants, and analyzed their Ca-binding properties and regulatory effects on actomyosin-tropmyosin Mg-ATPase activity. The results suggest that Ca binding to site IV is responsible for regulation at relatively low Ca concentrations, while site II has an essential role in full activation at higher Ca concentrations.