![]() Golgi resident transmembrane proteins include components involved in membrane traffic as well as enzymes involved in the glycosylation and proteolytic processing of cargo molecules ( Banfield, 2011). Newly synthesized cargo proteins arrive at the Golgi from the ER, advance from early to late cisternae, and ultimately depart from the TGN to either the plasma membrane or the endosomal/lysosomal/vacuolar system ( De Matteis and Luini, 2008). Despite these differences in organization, the basic functions of the Golgi are conserved. In other organisms such as the budding yeast Saccharomyces cerevisiae, the cisternae are not stacked ( Mowbrey and Dacks, 2009 Papanikou and Glick, 2009). ![]() In many cell types, including animal and plant cells, the Golgi consists of stacks of disk-like cisternae ( Farquhar and Palade, 1981). The structure and composition of the Golgi apparatus are well described. This insight can explain the polarized distribution of transmembrane proteins in the Golgi. We infer that the AP-1/Ent5 pair mediates two sequential intra-Golgi recycling pathways that define two classes of Golgi proteins. Unexpectedly, various AP-1/Ent5–dependent Golgi proteins show either intermediate or late kinetics of residence in maturing cisternae. This recycling can be detected by removing AP-1 and Ent5, thereby diverting the AP-1/Ent5–dependent Golgi proteins into an alternative recycling loop that involves traffic to the plasma membrane followed by endocytosis. Here, we demonstrate that AP-1 cooperates with the Ent5 clathrin adaptor to recycle a set of Golgi transmembrane proteins, including some that were previously thought to pass through endosomes. We recently proposed that the AP-1 clathrin adaptor mediates intra-Golgi recycling late in the process of cisternal maturation. This question was addressed using the yeast Saccharomyces cerevisiae, in which the maturation of individual Golgi cisternae can be visualized. The pathways of membrane traffic within the Golgi apparatus are not fully known.
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