High impact information on RSR1

• We have identified a gene, BUD5, that resembles BUD1 and BUD2 in being required for both patterns; bud5- mutants also exhibit random budding in all cell types [1].
• We have identified four genes, BUD1-BUD4, necessary for the axial pattern by isolating mutants of alpha cells that do not exhibit this pattern [2].
• Mutants defective in BUD1, BUD2 or BUD5 choose bud sites randomly [3].
• Here we report that Bud2p is a GTPase-activating protein (GAP) for Bud1p with a sequence similar to the catalytic domain of rasGAPs, and that Bud2p purified from yeast stimulates GTP hydrolysis by Bud1p [3].
• Furthermore, the simultaneous disruption of RAS1, RAS2, and the RAS-related gene RSR1 was lethal at any temperature [4].

Biological context of RSR1

• Two other bud site selection genes, RSR1, encoding a small GTPase, and BUD3, were also required for efficient cell fusion [5].
• The axl1 and rsr1 mutants exhibited defects in pheromone-induced morphogenesis [5].
• Electron microscopy of axl1, rsr1, and fus2 prezygotes revealed similar defects in nuclear migration, vesicle accumulation, cell wall degradation, and membrane fusion during cell fusion [5].
• RSR1 maps between CDC62 and ADE3 on the right arm of chromosome VII; its predicted product is approximately 50% identical to other proteins in the ras family [6].
• In the budding yeast Saccharomyces cerevisiae, the Ras-like GTPase Bud1/Rsr1 and its guanosine 5'-diphosphate (GDP)/guanosine 5'-triphosphate (GTP) exchange factor Bud5 are involved in the selection of a specific site for growth, thus determining cell polarity [7].

Anatomical context of RSR1

• Like Ras, Bud1p GTPase is constitutively associated with the plasma membrane; however, concentrated activities of Bud5p GDP-GTP exchange factor and Bud2p GTPase-activating protein at the future bud site promote rapid cycling of Bud1p between GTP- and GDP-bound conformations in a spatially restricted manner [8].
• Consistent with this hypothesis, we found an Rsr1p-specific GTPase-activating protein (GAP) activity in yeast membranes which was not active toward p21rap1A, indicating that p21rap1A may be predominantly GTP bound in yeast cells [9].
• In this paper we demonstrate that the intrinsic GTPase activity of the Rsr1 protein is stimulated by GAP3 purified from bovine brain cytosol [10].
• Surprisingly, Rsr1/Bud1 also localized to intracellular membranes [11].
• Similarly, in activated neutrophils, the NADPH oxidase is found in a complex with rap1, the mammalian homologue of BUD1 (BoKoch et al., 1989) [12].

Associations of RSR1 with chemical compounds

• Thus the lysine repeat and the CAAX motif of Rsr1/Bud1 are important for its localization to the plasma membrane and to the polarized growth sites [11].

Physical interactions of RSR1

• We also show that Cdc42p coimmunoprecipitated with Rsr1p/Bud1p from yeast extracts [13].
• Rsr1p/Bud1p binds to the CH-domain of Cdc24p, which is essential for its function in vivo [14].
• It seems likely, therefore, that the target is not simply a plasma-membrane protein but may be a complex of proteins whose formation is under the control of the rap1/BUD1 GTPase [12].

Regulatory relationships of RSR1

• Rsr1p/Bud1p appears to activate the GEF activity of Cdc24p in vivo, possibly by triggering a conformational change that dissociates the PB1-domain from its intramolecular binding site [14].
• Although Rap1-specific GAP stimulated the GTPase of Rsr1p in vitro, it did not dominantly interfere with Rsr1p function in vivo [9].
• The relative timing of Bud5 and Bud2 localizations suggests that both regulators contribute to the spatially specific control of Bud1 GTPase [15].

Other interactions of RSR1

• The RSR1 gene, which was previously identified as a multicopy suppressor of Ts- mutations in the bud-emergence gene CDC24, encodes a GTPase of the Ras family that is required for both budding patterns [16].
• Here we show specific genetic interactions between RSR1/BUD1 and particular cdc42 mutants defective in polarity establishment [13].
• RESULTS: Here we show that a rsr1 gic1 gic2 mutant fails to initiate budding, resulting in unbudded, large, and multinucleated cells [17].
• This suppression by BUD5 can be reversed by simultaneous overexpression of RNA1, and is not Rsr1p-dependent, nor allele-specific [18].
• Bud formation in yeast involves the actions of the Ras-type GTPase Rsr1, which is required for the proper selection of the bud site, and the Rho-type GTPase Cdc42, which is necessary for the assembly of cytoskeletal structures at that site [19].

Analytical, diagnostic and therapeutic context of RSR1

• Immunofluorescence and cell fractionation indicate that Bud1p remains associated with the membrane throughout its GTPase cycle [8].

References