Sst2 is a GTPase-activating protein for Gpa1:: Purification and characterization of a cognate RGS-Gα protein pair in yeast

Genetic studies in the yeast Saccharomyces cerevisiae have shown that SST2 promotes pheromone desensitization in vivo. Sst2 is the founding member of the RGS (regulators of G protein signaling) family of proteins, which in mammals act as GAPs (GTPase activating proteins) for several subfamilies of G alpha proteins in vitro. A similar activity for Sst2 has not been demonstrated, and it is not self-evident from sequence homology arguments alone. Here we describe the purification of Sst2 and its cognate Get protein (Gpa1) in yeast, and demonstrate Sst2-stimulated Gpa1 GTPase activity. His-tagged versions of Sst2 and Gpa1 were expressed in E. coli, and purified using Ni2+-agarose and ion exchange chromatography. Time-course binding experiments reveal that Sst2 does not affect the binding or release of guanine nucleotides. Similarly, steady-state GTPase assays reveal that Sst2 does not alter the overall rate of hydrolysis, including the rate-limiting nucleotide exchange step. Single-turnover GTPase assays reveal, however, that Sst2 is a potent stimulator of GTP hydrolysis. Sst2 also exhibits GAP activity for mammalian G(o) alpha, and the mammalian RGS protein GAIP exhibits GAP activity for Gpa1. Finally, we show that Sst2 binds with highest affinity to the transition state of Gpa1 (GDP-AlF4--bound), and with much lower affinity to the inactive (GDP-bound) and active (GTP gamma S-bound) conformations. These experiments represent the first biochemical characterization of Gpa1 and Sst2, and provide a molecular basis for their well-established biological roles in signaling and desensitization.