FARNESYLATION OF P21 RAS PROTEINS IN XENOPUS OOCYTES

Unprocessed p21 Ras proteins microinjected into Xenopus oocytes were r adiolabeled by coinjected [H-3]farnesyl pyrophosphate, a direct farnes yl donor substrate for all known mammalian farnesyl-transferases. Mevi nolin, an inhibitor of HMG CoA reductase which reduces the levels of m evalonate and thus farnesyl pyrophosphate, blocked oncogenic H-Ras(va1 12) induced germinal vesicle breakdown in oocytes. This mevinolin caus ed block was completely reversed by co-injected farnesyl pyrophosphate . The putative farnesyltransferase in Xenopus oocytes was identified t o be similar to those found in mammalian cells in that it requires an intact CAAX box motif in addition to the conserved cysteine residue at the fourth position from the C-terminus of Pas proteins for its farne sylating activity. Peptide inhibitors of farnesyltransferase such as C VIM and TKCVIM were shown to inhibit farnesylation of microinjected Ra s proteins thereby blocking its function namely the induction of oocyt e maturation. These results demonstrate that Xenopus oocytes process b acterially produced mammalian Pas proteins in a manner similar to, if not identical with that in mammalian cells, thus validating the contin ued use of the Xenopus oocyte system for unraveling the functions of P as proteins. Furthermore, our results indicate that the oocyte system may be a useful in vivo model for studying the farnesylation of human Ras proteins, its regulation, and the effects of farnesyltransferase i nhibitors.