Xenopus oocytes have become an ideal system for studies of signal transduction pathways, especially for the function studies of ras proteins. In order to use Xenopus oocytes as a model system, it is important to characterize the structures of Xenopus proteins in which we are interested in our own studies. Previously, the cDNAs encoding for protein kinase C (PKC), PI-specific phospholipase C (PLC), and various G-proteins have been molecularly cloned from the mammalian species. Using the mammalian cDNAs as the probes, Xenopus laevis cDNA clones, encoding for Go alpha, Gi alpha, ADP-ribosylation factor (ARF), ras, two types of PLC, and two types of PKC have been isolated from Xenopus oocytes in our laboratory. These cDNA clones have been sequenced, and comparison of the sequences of the coding regions with mammalian cDNAs shows good homology at the nucleotide and deduced amino acid level. The conserved nature of amino acid sequences suggests that the proteins are functionally interchangeable among different biological systems. Because Xenopus is evolutionarily closer to mammalian species than yeast, the Xenopus oocyte system would be a better choice for the functional studies of mammalian proteins in signal transduction. We have purified mammalian ras, ras GTPase activating protein (GAP), ARF, and PKC, and studied their functions in Xenopus oocytes. Our findings are summarized as follows: 1. GAP induces a rapid increase of diacylglycerol (DG) and GAP action is downstream or independent of ras functions in the stimulation of DG formation; 2. ARF has an antagonistic effect on ras actions; and 3. PKC potentiates ras- and insulin-induced oocyte maturation and ribosomal protein S6 phosphylation. Although our research efforts have been focused on ras-associated functions in Xenopus oocytes, we have also studied the regulation of oocyte maturation by other biological response modifiers (BRMs). For example, we have shown the negative regulation of progesterone-induced oocyte maturation by prostaglandin El or E2. Biochemical mechanisms of the inhibitory effects are currently under investigation.
