Akazara scallop (Chlamys nipponensis akazara) troponin C (TnC) of striated adductor muscle binds only one Ca2+ ion at the C-terminal EF-hand motif (Site IV), but it works as the Ca2+-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 Globe (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 Ca2+-regulatory mechanism of muscle contraction by scallop Tn, we have analyzed the Ca2+ -binding properties of the complex of TnC Globe and TnI peptide, and their interaction using isothermal titration microcalorimetry, nuclear magnetic resonance, circular dichroism, and gel filtration chromatography. The results showed that single Ca2+-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 Globe of scallop TnC. We therefore assumed that the effect of Ca2+-binding must lead to a change in the interaction mode between the Globe of TnC and the TnI peptide. The change should be the first event of the transmission of Ca2+ signal to TnI in Tit ternary complex. (c) 2007 Elsevier Inc. All rights reserved.