SEPARATE ROLES FOR N-TERMINI AND C-TERMINI OF THE STE4 (BETA)-SUBUNITOF THE SACCHAROMYCES-CEREVISIAE G-PROTEIN IN THE MEDIATION OF THE GROWTH ARREST - LACK OF GROWTH-ARRESTING ACTIVITY OF MAMMALIAN BETA-GAMMACOMPLEXES

Mating pheromone signal transduction in Saccharomyces cerevisiae invol ves a G protein composed to Scg1p (Gpa1p), Ste4p and Ste18p subunits, homologous to the alpha, beta and gamma subunits of mammalian G protei ns. Growth arrest in G1 phase is activated by the Ste4p/Ste18p complex via a downstream pathway and it is negatively controlled by the Scg1p subunit. Here we explored whether mammalian beta or gamma subunits co uld functionally substitute for their yeast homologues. While no evide nce was obtained for functional replacement of Ste4p and Ste18p, we fo und that overexpression of Ste18p potentiated the effect of hybrid pro teins in which the N terminus of the Ste4p subunit was replaced by tha t of the mammalian beta. ste4 mutants having deletions in the N termin us showed a decreased activity in signalling to the downstream effecto r of the pheromone response. This defect was totally cured by overexpr ession of Ste18p, indicating that the truncated forms of Ste4p have re tained their ability to form an active complex with Ste18p. Removal of six amino acids from the C terminus of Ste4p rendered a completely in active subunit and this defect persisted in hybrids where the C termin us was placed by that of the beta subunit, indicating that the C termi nus of Ste4p is essential to trigger the effector of the yeast pheromo ne response pathway.