The three-dimensional structure of Ca2+-bound calcyclin: implications for Ca2+-signal transduction by S100 proteins.

Background: Calcyclin is a member of the S100 subfamily of EF-hand Ca2+-binding proteins. This protein has implied roles in the regulation of cell growth and division, exhibits deregulated expression in association with cell transformation, and is found in high abundance in certain breast cancer cell lines. The novel homodimeric structural motif first identified for apo calcyclin raised the possibility that S100 proteins recognize their targets in a manner that is distinctly different from that of the prototypical EF-hand Ca2+ sensor, calmodulin. The NMR solution structure of Ca2+-bound calcyclin has been determined in order to identify Ca2+-induced structural changes and to obtain insights into the mechanism of Ca2+-triggered-target protein recognition. Results: The three-dimensional structure of Ca2+-bound calcyclin was calculated with 1372 experimental constraints, and is represented by an ensemble of 20 structures that have a backbone root mean square deviation of 1.9 Angstrom for the eight helices. Ca2+-bound calcyclin has the same symmetric homodimeric fold as observed for the apo protein. The helical packing within the globular domains and the subunit interface also change little upon Ca2+ binding, A distinct homology was found between the Ca2+-bound states of the calcyclin subunit and the monomeric S100 protein calbindin D-9k. Conclusions: Only very modest Ca2+-induced changes are observed in the structure of calcyclin, in sharp contrast to the domain-opening that occurs in calmodulin and related Ca2+-sensor proteins. Thus, calcyclin, and by inference other members of the S100 family, must have a different mode for transducing Ca2+ signals and recognizing target proteins. This proposal raises significant questions concerning the purported roles of S100 proteins as Ca2+ sensors.