Fig. 7. Role of AP-1 and Fab1p in GFP-CPS trafficking. Potential AP-1-dependent trafficking steps are shown as bold arrows; adapted from Black and Pelham, and Hinners and Tooze (Black and Pelham, 2000; Hinners and Tooze, 2003). In wild type-cells cargo traffic to the EE, are ubiquitylated by Rsp5p and, on reaching the MVB, are sorted into ILVs. The MVB fuses with the vacuole, releasing vesicles and cargo into the vacuole lumen. In cells where AP-1 and Fab1p function is compromised, cargo spill over into the GGA pathway, bypassing the EE, and arrive at the MVB un-ubiquitylated. As a result, cargoes fail to be recognised by the MVB sorting machinery and are delivered to the limiting membrane of the vacuole. In the absence of AP-1 and Fab1p function some cargo could also be trafficked to the plasma membrane and be ubiquitylated by Rsp5p; this could explain why there is ubiquitylation of cargo proteins in fab1 cells (Katzmann et al., 2004; Reggiori and Pelham, 2002).