COPI-dependent intra-Golgi recycling at an intermediate stage of cisternal maturation

The traffic pathways that recycle resident Golgi proteins during cisternal maturation are not completely defined. We addressed this challenge using the yeast Saccharomyces cerevisiae, in which maturation of individual cisternae can be visualized directly. A new assay captures a specific population of Golgi-derived vesicles at the bud neck, thereby revealing which resident Golgi proteins are carried as cargo in those vesicles. This method supplies evidence for at least three classes of intra-Golgi vesicles with different cargo compositions. Consistent with our previously published data, one class of vesicles mediates a late pathway of intra-Golgi recycling with the aid of the AP-1 and Ent5 clathrin adaptors, and a second class of vesicles mediates an early pathway of intra-Golgi recycling with the aid of the COPI vesicle coat. Here, we identify another COPI-dependent pathway of intra-Golgi recycling and show that it operates kinetically between the two previously known pathways. Thus, intra-Golgi recycling is mediated by multiple COPI-dependent pathways followed by a clathrin-dependent pathway.