Ypts and TRAPPs in Golgi Dynamics
The conserved Ypt/Rab GTPases regulate the pathways of intracellular transport in eukaryotic cells. They accomplish this regulation in conjunction with their activators, guanine exchange factors (GEFs). Yeast Ypt1 (mammalian Rab1) and Ypt31/32 (mammalian Rab11) are essential for ER-to-Golgi and Golgi-to-PM trafficking, respectively. However, regulation of intra-Golgi processes, such as Golgi cisternal progression, by these Ypt/Rabs remains poorly understood. In this thesis, I report findings on Ypt1 and Ypt31 as well as their GEFs, the TRAPPI and TRAPPII complexes, within the Golgi. Using live-cell microscopy and immunofluorescence, I establish that Ypt1 and Ypt31 polarize to opposite ends of the Golgi, early and late, respectively. They co-localize on a compartment that contains both early and late Golgi proteins, which I termed transitional. Furthermore, using live-cell and time-lapse microscopy, I show that Ypt1 and Ypt31 regulate two distinct Golgi cisternal progression steps, early-to-transitional and transitional-to-late, respectively. Correspondingly, I provide evidence that the TRAPPII complex has a similar pattern of Golgi compartmental localization as Ypt31 and not of Ypt1. Together, these results show novel regulation for Golgi cisternal maturation by Ypt/Rab GTPases, clears up controversy for the placement of Ypt1 and Ypt31 to specific Golgi compartments, and places TRAPPII as the GEF for Ypt31 in vivo.