The TRAPPIII complex activates the GTPase Ypt1 (Rab1) in the secretory pathway.

Rab GTPases serve as molecular switches to regulate eukaryotic membrane trafficking pathways. The transport protein particle (TRAPP) complexes activate Rab GTPases by catalyzing GDP/GTP nucleotide exchange. In mammalian cells, there are two distinct TRAPP complexes, yet in budding yeast, four distinct TRAPP complexes have been reported. The apparent differences between the compositions of yeast and mammalian TRAPP complexes have prevented a clear understanding of the specific functions of TRAPP complexes in all cell types. In this study, we demonstrate that akin to mammalian cells, wild-type yeast possess only two TRAPP complexes, TRAPPII and TRAPPIII. We find that TRAPPIII plays a major role in regulating Rab activation and trafficking at the Golgi in addition to its established role in autophagy. These disparate pathways share a common regulatory GTPase Ypt1 (Rab1) that is activated by TRAPPIII. Our findings lead to a simple yet comprehensive model for TRAPPIII function in both normal and starved eukaryotic cells.