ISOLATED SOS1 PH DOMAIN EXHIBITS GERMINAL VESICLE BREAKDOWN-INDUCING ACTIVITY IN XENOPUS OOCYTES

Purified, bacterially expressed PH domains of Sos1, IRS-1, beta ARK, a nd PLC delta(1) were analyzed functionally by means of microinjection into full grown, stage VI Xenopus laevis oocytes, Whereas the PH domai ns from IRS-1, beta ARK, or PLC delta(1) did not show any effect in th e oocytes, injection of the purified Sos1 PH domain resulted in induct ion of significant rates of germinal vesicle breakdown and meiotic mat uration, Furthermore, the Sos1 PH domain exhibited also significant sy nergy with insulin or coinjected normal Ras protein in induction of ge rminal vesicle breakdown, although it did not affect the rate of proge sterone-induced maturation. These results suggest that purified, isola ted PH domains retain, at least in part, their functional specificity and that Xenopus oocytes may constitute a useful biological system to analyze the functional role of the Sos1 PH domain in Ras signaling pat hways.