Required for mating and morphogenesis. May contain a cryptic binding site for cdc42 that is enhanced by binding Ras. Interacts directly with scd2. Promotes the exchange of cdc42-bound GDP by GTP. Involved in septation and stimulates the elongation of conjugation tubes. Scd1, scd2, cdc42, and ras1, in its GTP-bound state, act cooperatively to form a protein complex. Interacts with moe1 and cdc42. Located at the growing cell ends and tips of conjugation tubes.