GDP-GTP exchange factor (GEF) for the small GTPase BUD1/RSR1. Regulates the activity of BUD1 together with BUD2 which is a GTPase-activating protein (GAP) of BUD1. Required to produce both the axial and bipolar patterns of bud site selection. Determines the orientation of division axis. Overexpression can suppress mutations in PRP20 which is the GEF for GSP1. May be a cytoplasmic GEF for GSP1. Might also act on the Ras-like protein CDC42. Appears to bind to Ras proteins but not to activate them. Interacts with AXL2, BEM1, GSP1 and in haploid cells with AXL1. In haploid cells, forms double rings that encircle the mother-bud neck and split upon cytokinesis, each progeny cell inheriting BUD5 at the axial division remnant. In diploid cells, localizes to the poles of the cell. Present with 2110 molecules/cell in log phase SD medium.