Activation of ras p21 transforming properties associated with an increase in the release rate of bound guanine nucleotide.

An Ala-to-Thr substitution at position 59 activates the transforming properties of the p21ras protein without impairment of GTPase activity, a biochemical alteration associated with other activating mutations. To investigate the basis for the transforming properties of the Thr-59 mutant, we characterized guanine nucleotide release. This reaction exhibited a slow rate and stringent temperature requirements. To further dissect the release reaction, we used monoclonal antibodies directed against different epitopes of the p21 molecule. One monoclonal specifically interfered with nucleotide release, while others which recognized different regions of the molecule blocked nucleotide binding. Mutants with the Thr-59 substitution exhibited a three- to ninefold-higher rate of GDP and GTP release than normal p21 or mutants with other activating lesions. This alteration in the Thr-59 mutant would have the effect of increasing its rate of nucleotide exchange. In an intracellular environment with a high GTP/GDP ratio, this would favor the association of GTP with the Thr-59 mutant. Consistent with knowledge of known G-regulatory proteins, these findings support a model in which the p21-GTP complex is the biologically active form of the p21 protein.