Effects of ions on the intrinsic activities of c‐H‐ras protein p21

The influence of the ionic environment on the intrinsic GTPase activity and the guanine-nucleotide interaction of Ha-ras protein p21 were studied in various experimental conditions and compared with the behaviour of elongation factor (EF) Tu. To this purpose, nucleotide-free p21 was prepared, which is much more stable than by any other reported method. Specific differences between p21 and EF-Tu were found in the action of divalent anions which strongly enhance the dissociation rate of p21.GDP without affecting that of EF-Tu. Unlike EF-Tu, the GTPase activity of p21 is only slightly dependent on the presence and concentration of monovalent cations. The concentrations of Mg2+ influencing the dissociation rate of the p21.GDP complex are much higher than for the intrinsic GTPase activity, an effect also observed for EF-Tu. These results point to two distinct roles of Mg2+: as a conformational regulator of the interaction with the substrate and as a key element for the hydrolysis of GTP. The GTPase activity of p21 is not affected by changes in pH over the range 6-9.2, different from that of EF-Tu. However, stabilization by kirromycin confers a pH independence to the GTPase of EF-Tu in the pH range 6.5-10, suggesting that the bell-shaped behaviour of this activity in the absence of the antibiotic is due to denaturation. This implies similar properties in the catalytic mechanism of these two guanine-nucleotide-binding proteins.