Structural differences between valine-12 and aspartate-12 Ras proteins maymodify carcinoma aggression

Recent evidence associates the codon 12 valine-for-glycine (G12V) mutant Ki -Ras protein with higher stage and increased lethality of colorectal carcin omas, while the codon 12 aspartate-far-glycine (G12D) Ras mutation shows no such association. Several observations may be relevant to this phenomenon, First, GTPase activity of G12Y Ras is one-quarter that of G12D Ras and one -tenth that of wild-type (WT) Pas. Second, binding of the GTP analogue GppN p to G12D Ras is 8-fold weaker than its binding to G12V or WT Ras and cryst al structures indicate that electrostatic repulsion between the carboxylate group of the G12D Asp-12 side-chain and the gamma phosphate of the bound n ucleotide may make GTP binding to G12D Pas weaker even than that of GppNp, It is proposed that this lowering of affinity for GTP allows G12D Pas an es cape From the oncogenic GTP-bound state, whereas GTP tightly bound to G12V mutant Ras generates a more persistent, potentially oncogenic, signal. Stru ctural comparisons also suggest that differences between the Switch I (effe ctor) region of G12D and G12V Pas could modify interactions with downstream signalling molecules such as Raf-l, neurofibromin, and phosphatidylinosito l 3-hydroxy-kinase. Other differences between the G12D and G12V mutant Ras proteins include a lower affinity of the GTPase activating protein GAP for G12V than for G12D or WT Ras; but, as both G12D and G12V Pas are refractory to GTPase activation by GAP binding, this may be less significant, These s tudies complement experimental data showing that such Ras mutations differ in their effects in vitro and in vivo and, with recent data indicating hete rogeneity of uas mutation in colorectal carcinomas and other tumours, make it plausible that codon 12 Pas mutations differ in carcinogenic potential a nd prognostic significance. Copyright (C) 1999 John Wiley & Sons, Ltd.