Direct interaction between a myosin V motor and the Rab GTPases Ypt31/32 is required for polarized secretion.

Rab GTPases recruit myosin motors to endocytic compartments, which in turn are required for their motility. However, no Ypt/Rab GTPase has been shown to regulate the motility of exocytic compartments. In yeast, the Ypt31/32 functional pair is required for the formation of trans-Golgi vesicles. The myosin V motor Myo2 attaches to these vesicles through its globular-tail domain (GTD) and mediates their polarized delivery to sites of cell growth. Here, we identify Myo2 as an effector of Ypt31/32 and show that the Ypt31/32-Myo2 interaction is required for polarized secretion. Using the yeast-two hybrid system and coprecipitation of recombinant proteins, we show that Ypt31/32 in their guanosine triphosphate (GTP)-bound form interact directly with Myo2-GTD. The physiological relevance of this interaction is shown by colocalization of the proteins, genetic interactions between their genes, and rescue of the lethality caused by a mutation in the Ypt31/32-binding site of Myo2-GTD through fusion with Ypt32. Furthermore, microscopic analyses show a defective Myo2 intracellular localization in ypt31Delta/32ts and in Ypt31/32-interaction-deficient myo2 mutant cells, as well as accumulation of unpolarized secretory vesicles in the latter mutant cells. Together, these results indicate that Ypt31/32 play roles in both the formation of trans-Golgi vesicles and their subsequent Myo2-dependent motility.