Three-dimensional structure of a sarcoplasmic calcium-binding protein from Nereis diversicolor

The three-dimensional structure of a sarcoplasmic Ca2(+)-binding protein from the sandworm Nereis diversicolor has been determined at 3.0 A resolution using multiple isomorphous replacement techniques. The NH2-terminal half of the molecule contains one variant Ca2(+)-binding domain with a novel helix-loop-helix conformation and one Ca2(+)-binding domain that is no longer functional because of amino acid changes. The overall conformation of this pair of domains is different from any previously described Ca2(+)-binding protein. The COOH-terminal half of the protein contains two Ca2(+)-binding domains with the usual helix-loop-helix configuration and is similar to calmodulin and troponin C. Unlike calmodulin or troponin C, there is no exposed alpha-helix connecting the two halves of the molecule, so the overall structure is much more compact.