Different domains of the AP-1 adaptor complex are required for Golgi membrane binding and clathrin recruitment

The assembly of clathrin-coated buds on the Golgi requires the recruitment of the heterotetrameric AP-1 adaptor complex, which is dependent on both guanine nucleotides and the small GTP-binding protein ADP-ribosylation factor (ARF). Here, we have investigated the structural domains of the AP-1 complex necessary for ARF-mediated translocation of the adaptor complex onto Golgi membranes and the subsequent recruitment of clathrin onto the membrane. Controlled proteolysis of purified AP-1, derived from bovine adrenal coated vesicles, was used to generate AP-1 core fragments composed of the amino- terminal trunk regions of the β1 and γ subunits and associated μ1 and σ1 subunits, and lacking either the β1 subunit carboxyl-terminal appendage or both β1 and γ, subunit appendages. On addition of these truncated fragments to AP-1-depleted adrenal cytosol, both types of core fragments were efficiently recruited onto Golgi membranes in the presence of GTPγS. Recruitment of both core fragments was inhibited by the fungal metabolite brefeldin A, indicative of an ARF-dependent process. Limited tryptic digestion of recruited, intact cytosolic AP-1 resulted in the quantitative release of the globular carboxyl-terminal appendage domains of the β1 and γ subunits. The adaptor core complex remained associated with the Golgi membranes. Recruitment of cytosolic clathrin onto the Golgi membranes was strictly dependent on the presence of intact AP-1. Tryptic removal of the β1 subunit appendage prevented subsequent clathrin recruitment. We conclude that the structural determinants required for the ARF-mediated binding of cytosolic AP-1 onto Golgi membranes are contained within the adaptor core, and that the carboxyl-terminal appendage domains of the and y subunits do not play any role in this process. Subsequent recruitment of cytosolic clathrin, however, requires an intact β1 subunit.