Results: The three-dimensional solution structure of Ca2+-loaded protein S has been determined using multi-dimensional heteronuclear NMR spectroscopy. (Sixty structures were calculated, from which thirty were selected with a root mean square difference from the mean of 0.83 angstrom for backbone atoms and 1.22 angstrom for all non-hydrogen atoms.) The structure was analyzed and compared in detail with X-ray crystallographic structures of betagamma-crystallins. The two internally homologous domains of protein S were compared, and hydrophobic cores, domain interfaces, surface ion pairing, amino-aromatic interactions and potential modes of multimerization are discussed.

Conclusions: Structural features of protein S described here help to explain its overall thermostability, as well as the higher stability and Ca2+ affinity of the amino-terminal domain relative to the carboxy-terminal domain. Two potential modes of multimerization are proposed involving cross-linking of protein S molecules through surface Ca2+-binding sites and formation of the intramolecular protein S or gammaB-crystallin interdomain interface in an intermolecular context. This structural analysis may also have implications for Ca2+-dependent cell-cell interactions mediated by the vertebrate cadherins and Dictyostelium discoideum protein gp24.