Fig. 2. Hypothetical model for the cooperation of GGA proteins and AP-1 during clathrin coat recruitment on the TGN. (A) Association of the GGA GAT domain with ARF-GTP causes an inhibition of the ARF-GAP and allows enhanced recruitment of AP-1. (B) During coat recruitment AP-1 interacts with both ARF-GTP and GGA. Phosphorylation of cargo by CK2 increases the interaction of the VHS domain of the GGAs, as well as AP-1 with cargo. (C) Subsequent phosphorylation of the GGA proteins allows dissociation of GGAs and activation of GAP activity. Budding of AP-1-coated vesicles then proceeds from the TGN. Please note, for simplicity, clathrin, the ß-ears and -ears were omitted from the figure. Recently, it has been reported that µ1 and µ2 are phosphorylated by an adaptor-associated kinase called AAK1 (Ricotta et al., 2002). GAK (cyclin G-associated protein kinase) also has been shown to phopshorylate µ1 and µ2 (Umeda et al., 2000). Although it has been demonstrated that phosphorylation of µ2 by AAK1 fulfils a regulatory function in receptor-mediated endocytosis (Olusanya et al., 2001; Ricotta et al., 2002), the physiological significance of µ1 phosphorylation remains to be elucidated.