The RAS gene is found activate form in many different forms of human cancer and is clearly part of an oncogenic mechanism that triggers cell proliferation. However the functions of the Ras protein and its role in the process of oncogenesis are not understood. The normal cellular function of Ras has been the subject of intense investigation in many organisms including the yeast Saccharomyces cerevisiae: molecular and classical genetic studies in this organism have shown that one function of Ras is to interact with adenylate cyclase, controlling cAMP levels and thus cAMP-dependent kinase activity. The study of strains mutant in the RAS / cAMP pathway suggests that cell growth and division in response to nutrient availability is controlled through this cascade of reactions. However, as Ras does not control adenylate cyclase in higher organisms this pathway cannot be part of the oncogenic mechanism; alternative functions of Ras are the subject of intensive investigation. Yeast cells containing an activated RAS allele do not grow at normal rates on various nitrogen sources and are sensitive to amino acid analogs. Preliminary studies on the level of general amino acid permease activity in mutant RAS strains has shown thai the control of permease activity by ammonia is altered in these cells. ras2 mutants, in which no active Ras protein is made, have high GAP activity in all media, whereas, dominant activated RAS alleles had lowered activity (approx. 3x) even in derepressing medium. These altered transport rates may account for the growth phenotypes of RAS mutant strains. A thorough investigation of the effect of RAS and cAMP pathway mutants on the control of amino acid transport and metabolism in response to ammonia, and other nitrogen sources, is proposed. These assays are designed to determine the mechanism of coordination between general signals for cell growth and the transport of nitrogen compounds into the cell.
NIH ExPorter
NIH ExPorter