BACKGROUND: Calcyclin is a member of the S100 subfamily of EF-hand Ca(2+)-bindingproteins. This protein has implied roles in the regulation of cell growth anddivision, exhibits deregulated expression in association with celltransformation, and is found in high abundance in certain breast cancer celllines. The novel homodimeric structural motif first identified for apo calcyclin raised the possibility that S100 proteins recognize their targets in a mannerthat is distinctly different from that of the prototypical EF-hand Ca2+ sensor,calmodulin. The NMR solution structure of Ca(2+)-bound calcyclin has beendetermined in order to identify Ca(2+)-induced structural changes and to obtaininsights into the mechanism of Ca(2+)-triggered target protein recognition.RESULTS: The three-dimensional structure of Ca(2+)-bound calcyclin was calculatedwith 1372 experimental constraints, and is represented by an ensemble of 20structures that have a backbone root mean square deviation of 1.9 A for the eighthelices. Ca(2+)-bound calcyclin has the same symmetric homodimeric fold asobserved for the apo protein. The helical packing within the globular domains andthe subunit interface also change little upon Ca2+ binding. A distinct homologywas found between the Ca(2+)-bound states of the calcyclin subunit and themonomeric S100 protein calbindin D9k. CONCLUSIONS: Only very modestCa(2+)-induced changes are observed in the structure of calcyclin, in sharpcontrast to the domain-opening that occurs in calmodulin and relatedCa(2+)-sensor proteins. Thus, calcyclin, and by inference other members of theS100 family, must have a different mode for transducing Ca2+ signals andrecognizing target proteins. This proposal raises significant questionsconcerning the purported roles of S100 proteins as Ca2+ sensors.