Figure 9. Model for sorting of biosynthetic cargoes in the late Golgi.

(A) Sequential formation of GGA vesicles and AP-1 vesicles in yeast cells. The thick arrows represent progressive maturation of a Golgi cisterna over time. During the early-to-late Golgi transition of cisternal maturation, GGA adaptors arrive, and GGA vesicles that carry vacuolar cargoes (white squares) begin to form. Subsequently, the AP-1 adaptor arrives, and AP-1 vesicles that recycle resident Golgi proteins (not shown) as well as some secretory cargoes (black dots) begin to form. GGAs depart before AP-1 departs, but the formation phases for GGA vesicles and AP-1 vesicles overlap. (B) Comparison of Golgi structures in yeast and mammalian cells. In S. cerevisiae, the late Golgi or TGN stage accounts for about half of the maturation process, so the sequential arrival times and activities of GGAs and AP-1 are easy to detect. In mammalian cells, Golgi cisternae are stacked, with the youngest early Golgi cisterna at the opposite side of a stack from the oldest late Golgi/TGN cisterna. Only the trans-most cisterna of a mammalian Golgi stack functions as late Golgi/TGN, but during the lifetime of this cisterna, GGAs and AP-1 may arrive and act sequentially as in yeast.