Inhibition of g protein-mediated map kinase activation by members of a novel mammalian gene family

A general property of signal transduction pathways is that prolonged stimulation over time decreases responsiveness, a property termed desensitization. Yeast cells stimulated with mating pheromone activate a heterotnmenc G protein-linked, MAP kinase-dependent signaling pathway that induces G1 phase cell cycle arrest and morphologic differentiation. Eventually the cells desensitize to pheromone and resume growth. Genetic studies have revealed the relative importance of a desensitization mechanism that utilizes the SST2 gene product Sst2p. We have identified a novel mammalian gene family, termed RGS (Regulator of G Protein Signaling), that encodes structural and functional homologs of Sst2p. Four human RGSs have different molecular masses but share significant amino acid sequence homology, most notably within a 1 20 aa region termed the RGS domain. Some members are expressed predominantly in one tissue, such as RGS1 (B cells) and RGS4 (brain) while others are found in multiple tissues (RGS2,3). Southern blotting human genomic DNA with an oligonucleotide based on a highly conserved portion of the RGS domain predicted a minimum of 15 family members. Introduction of flGS family members into yeast blunts signal transduction through the pheromone response pathway; similar to SST2, they negatively regulate this pathway at the level of or upstream of the G protein. FtGS family members also markedly impair MAP kinase activation in mammalian cells. In 293T cells transfected with either lL-8 or muscarmir: m2 receptors, overexpression of RGSs inhibits MAP kinase activation in response to the natural ligand. Further, stimulation of endogenous G protein-coupled platelet-activating factor (PAF) receptors on a B eel! line induces RGS1 expression while constitutive expression of RGS1 in these cells impairs MAP kinase activation in response to PAF treatment. These data suggest the existence and likely importance ni an SST2-like desensitization in mammalian cells.