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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.
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2 chain. The appendages
of the ß2 and 
subunits, determined independently
(Owen et al., 2000