NMR-DERIVED 3-DIMENSIONAL SOLUTION STRUCTURE OF PROTEIN-S COMPLEXED WITH CALCIUM

Background: Protein S is a developmentally-regulated Ca2+-binding prot ein of the soil bacterium Myxococcus xanthus. It functions by forming protective, multilayer spore surface assemblies which may additionally act as a cell-cell adhesive. Protein S is evolutionarily related to v ertebrate lens betagamma-crystallins. Results: The three-dimensional s olution structure of Ca2+-loaded protein S has been determined using m ulti-dimensional heteronuclear NMR spectroscopy. (Sixty structures wer e 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 c ompared in detail with X-ray crystallographic structures of betagamma- crystallins. The two internally homologous domains of protein S were c ompared, and hydrophobic cores, domain interfaces, surface ion pairing , amino-aromatic interactions and potential modes of multimerization a re discussed. Conclusions: Structural features of protein S described here help to explain its overall thermostability, as well as the highe r stability and Ca2+ affinity of the amino-terminal domain relative to the carboxy-terminal domain. Two potential modes of multimerization a re proposed involving cross-linking of protein S molecules through sur face Ca2+-binding sites and formation of the intramolecular protein S or gammaB-crystallin interdomain interface in an intermolecular contex t. This structural analysis may also have implications for Ca2+-depend ent cell-cell interactions mediated by the vertebrate cadherins and Di ctyostelium discoideum protein gp24.