Akazara scallop (Chlamys nipponensis akazara) troponin C (TnC) of striated adductor muscle binds only one Ca 2+ ion at the C-terminal EF-hand motif (Site IV), but it works as the Ca 2+ -dependent regulator in adductor muscle contraction. In addition, the scallop troponin (Tn) has been thought to regulate muscle contraction via activating mechanisms that involve the region spanning from the TnC C-lobe (C-lobe) binding site to the inhibitory region of the TnI, and no alternative binding of the TnI C-terminal region to TnC because of no similarity between second TnC-binding regions of vertebrate and the scallop TnIs. To clarify the Ca 2+ -regulatory mechanism of muscle contraction by scallop Tn, we have analyzed the Ca 2+ -binding properties of the complex of TnC C-lobe and TnI peptide, and their interaction using isothermal titration microcalorimetry, nuclear magnetic resonance, circular dichroism, and gel filtration chromatography. The results showed that single Ca 2+ -binding to the Site IV leads to a structural transition not only in Site IV but also Site III through the structural network in the C-lobe of scallop TnC. We therefore assumed that the effect of Ca 2+ -binding must lead to a change in the interaction mode between the C-lobe of TnC and the TnI peptide. The change should be the first event of the transmission of Ca 2+ signal to TnI in Tn ternary complex.