RALGDS FUNCTIONS IN RAS-MEDIATED AND CAMP-MEDIATED GROWTH-STIMULATION

Thyroid-stimulating hormone stimulates proliferation through both the cAMP-dependent protein kinase and Ras but not through Raf-l and mitoge n-activated and extracellular signal-related kinase kinase. We now rep ort that thyroid-stimulating hormone represses mitogen-activated prote in kinase activity and that microinjection of an effector domain mutan t Ha-Ras protein, Ras(12V,37G), defective in Raf-l binding and mitogen -activated protein kinase activation, stimulates DNA synthesis in quie scent and thyroid-stimulating hormone-treated thyrocytes. A yeast two- hybrid screen identified RalGDS as a Ras(12V,37G) binding protein and therefore a potential effector of nas in these cells. Associations bet ween Ras and RalGDS were observed in extracts prepared from thyroid ce lls. Microinjection of a mutant Ra1A(28N) protein thought to sequester RaIGDS family members reduced DNA synthesis stimulated by Ras as well as cAMP-mediated DNA synthesis in two cell lines which respond to cAM P with mitogenesis. These results support the idea that RalGDS may be an effector of Ras in cAMP-mediated growth stimulation.