Characterization and site-directed mutagenesis of a low M(r) GTP-binding protein, ram p25, expressed in Escherichia coli.

The ram gene encodes a GTP-binding protein with a M(r) of 25,068 (Nagata, K., Satoh, T., Itoh, H., Kozasa, T., Okano, Y., Doi, T., Kaziro, Y., and Nozawa, Y. (1990) FEBS Lett. 275, 29-32). It has a putative effector domain very similar to that of yeast SEC4 protein, and shares 40% identity and 60% homology with it, respectively. In order to analyze the biochemical properties, ram cDNA was engineered and inserted into a bacterial expression vector; this allowed the production at a high level of soluble recombinant ram p25 in Escherichia coli. The purified ram p25 contained an equimolar amount of GDP. The purified protein bound approximately 1 mol of 35S]guanosine 5'-O-(thiotriphosphate) GTP gamma S)/mol of protein, with a Kd value of 120 nM. 35S]GTP gamma S binding to this protein was inhibited by GTP and GDP, but not by ATP and ADP. In the presence of 10 mM Mg2+, the dissociation of 8,5'-3H]GDP and 35S]GTP gamma S from ram p25 occurred with rates of 0.015 min-1 and 0.004 min-1, respectively, showing that the ram p25 has a higher affinity for GTP than GDP. The rate of release of Pi from gamma-32P]GTP-bound ram p25 was calculated to be 0.011 min-1. The contribution of guanine nucleotide-binding and GTP-hydrolysis domains of the protein to its biochemical activities was investigated by site-directed mutagenesis. Substitution of Val for Gly at position 19 resulted in disappearance of 35S]GTP gamma S- and 3H]GDP-binding activity in spite of good expression of the protein. Mutations of Thr41 to Ser, Ala76 to Thr, and Asn133 to His slightly increased the rates of 35S] GTP gamma S binding and 3H]GDP dissociation, but had almost no effects on the manner of gamma-32P]GTP hydrolysis. Replacement of Gln78 with Leu significantly increased the 3H]GDP dissociation rate (7-fold) and decreased GTP hydrolytic activity considerably.