Recruitment of AP-1 clathrin adaptors to liposomal membranes

Protein and membrane traffic between organelles within the endocytic and exocytic pathway is mediated most prominently by coated vesicles. These vesicles are formed by the assembly of cytosolic coat proteins onto the donor membrane, which deform it into a bud so that vesicles can pinch off. Clathrin with its associated adaptors, COPI and COPII are the three major coats. Various in vitro studies allowed insight into the mechanism of coat formation. COPI and COPII vesicle budding from chemically defined liposomes has been reconstituted in vitro, using pure coat compounds. Further, it has been demonstrated that cargo is sorted into these vesicles. The mechanism of clathrin-coated vesicle formation appears to be more complicated. The AP-1 clathrin adaptor is involved in vesicle formation at the transGolgi network and endosomes. This work presents an in vitro assay where AP-1 is recruited to peptidoliposomes, presenting covalently linked peptides corresponding to sorting signals. In this system, AP-1 recruitment depends on myristyolated ADP-ribosylation factor(ARF1), GTP or GMP-PNP, tyrosine signals and a small amount of phosphoinositides, most prominently phosphatidyl inositol 4,5bisphosphate. In such a minimal system AP-1 is recruited as a highmolecular weight complex indicating the formation of a precoat in the absence of clathrin. GTP hydrolysis, induced by ARF GTPase-activating protein(ARFGAP1), disassembled this complex. Further, AP-1 is able to enhance the GAP activity of ARFGAP1 on myristoylated ARF1, suggesting a regulatory function of GTP hydrolysis in early steps of coat recruitment. This work provides insights into the mechanism of AP-1 clathrin coat formation which might also be used to investigate the recruitment of other coats.