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Troponin C (TnC) is the Ca 2+ -sensing subunit of troponin that triggers the contraction of striated muscles. In scallops, the striated muscles consume little ATP energy in sustaining strong contractile forces. The N-terminal domain of TnC works as the Ca 2+  sensor in vertebrates, whereas scallop TnC uses the C-terminal domain as the Ca 2+  sensor, suggesting that there are differences in the mechanism of the Ca 2+ -dependent regulation of muscles between invertebrates and vertebrates. Here, we report the crystal structure of  Akazara  scallop ( Chlamys nipponensis akazara ) adductor muscle TnC C-terminal domain (asTnC C ) complexed with a short troponin I fragment (asTnI S ) and Ca 2+ . The electron density of a Ca 2+  ion is observed in only one of the two EF-hands. The EF-hands of asTnC C  can only be in the fully open conformation with the assistance of asTnI S . The number of hydrogen bonds between asTnC C  and asTnI S  is markedly lower than the number in the vertebrate counterparts. The Ca 2+  modulation on the binding between asTnC C  and asTnI S  is weaker, but structural change of the complex depending on Ca 2+  concentration was observed. Together, these findings provide a detailed description of the distinct molecular mechanism of contractile regulation in the scallop adductor muscle from that of vertebrates.

Structure of the Ca 2+ -saturated C-terminal domain of scallop troponin C in complex with a troponin I fragment