Cooperation of GGAs and AP-1 in Packaging MPRs at the Trans-Golgi Network

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Abstract
The Golgi-localized, γ-ear–containing, adenosine diphosphate ribosylation factor–binding proteins (GGAs) are multidomain proteins that bind mannose 6-phosphate receptors (MPRs) in the Golgi and have an essential role in lysosomal enzyme sorting. Here the GGAs and the coat protein adaptor protein–1 (AP-1) were shown to colocalize in clathrin-coated buds of the trans-Golgi networks of mouse L cells and human HeLa cells. Binding studies revealed a direct interaction between the hinge domains of the GGAs and the γ-ear domain of AP-1. Further, AP-1 contained bound casein kinase–2 that phosphorylated GGA1 and GGA3, thereby causing autoinhibition. This could induce the directed transfer of the MPRs from GGAs to AP-1. MPRs that are defective in binding to GGAs are poorly incorporated into AP-1–containing clathrin-coated vesicles. Thus, the GGAs and AP-1 interact to package MPRs into AP-1–containing coated vesicles.