UC San Diego

How does the Golgi apparatus maintain its organization amidst the constant flow of traffic through the secretory pathway? It was previously believed that cells maintain biochemically distinct membranes rather than constantly reproducing them, and that daughter cells inherit differentiated membrane from the mother. However, recent evidence suggests that the entire Golgi apparatus rapidly cycles through the Endoplasmic Reticulum (ER), indicating that Golgi membranes are constantly formed from ER membranes. We have tested this idea with a procedure that traps Golgi proteins in the ER when they visit there. Rapamycin induces a specific association between FKBP and FRAP. Golgi enzymes fused to FKBP can be captured in the ER when they visit there by an ER protein fused to FRAP in the presence of rapamycin. With this method the rate at which Golgi proteins associate with the ER can be measured while the secretory pathway remains intact. In Chapter I the ER-trapping procedure is utilized to test whether Golgi membranes fuse with the ER during cell division in mammalian cells. In mitotic cells Golgi membranes are broken down into small elements and then reformed in daughter cells. It had been reported that these small elements fuse with the ER, indicating that the Golgi is made de novo from the ER of each daughter cell. A sialyltransferase-FKBP reporter was not captured in the ER of dividing cells demonstrating that Golgi membranes remain separate from the ER during mitosis. Chapter II investigates the behavior of Golgi proteins in non- dividing cells. We found that, unlike a component of the ER-Golgi Intermediate Compartment (ERGIC) both early and late Golgi enzymes do not constitutively cycle through the ER. This combined with findings from Chapter I indicate that Golgi membranes are maintained independent of the ER. Chapter III the roles of GRASP65 in mitotic Golgi fragmentation and cell cycle progression are investigated. We discovered that phosphorylation of GRASP65 is required to initiate Golgi fragmentation and mitotic progression. We have also identified a 75 amino acid region of the protein that interacts with the factor(s) responsible for these events