CRYSTALLIZATION AND PRELIMINARY-X-RAY STRUCTURE-ANALYSIS OF THERMALLYUNSTABLE P21(H-RAS) GUANOSINE COMPLEXES

p21 is a small guanine nucleotide binding protein that is involved in intracellular signal transduction. Biochemical data suggest that the p resence of the beta-phosphate is essential for strong binding of guani ne nucleotides to the protein. Guanosine or GMP bind six orders of mag nitude more weakly to p21 than GDP or GTP. Moreover, the thermal stabi lity of the protein is dramatically reduced when bound to GMP or guano sine. We have crystallized C-terminally truncated forms of p21H-ras, w ith guanosine or GMP bound, in the space groups P4(3)2(1)2, P2(1)2(1)2 and P2(1). The crystals diffract in the range 2.8-2.2 angstrom. Detai ls of the crystallization procedures, the characterization of the crys tals and preliminary results of structure determination are described. An unexpected electron-density peak was found close to the position o f the beta-phosphate in the phosphate-binding loop.