THE SYNAPSE-SPECIFIC PHOSPHOPROTEIN-F1-20 IS IDENTICAL TO THE CLATHRIN ASSEMBLY PROTEIN AP-3

F1-20 and AP-3 are independently described, synapse-associated, develo pmentally regulated phosphoproteins with similar apparent molecular ma sses on SDS-polyacrylamide gel electrophoresis (PAGE). F1-20 was clone d and characterized because of its synapse specificity. AP-3 was purif ied and studied biochemically because of its function as a clathrin as sembly protein. Here we present evidence that establishes the identity of F1-20 and AP-3. Monoclonal antibodies against F1-20 and AP-3 both specifically recognize a single protein from mouse brain with an appar ent molecular mass of 190 kDa on SDS-PAGE. These monoclonal antibodies also specifically recognize the cloned F1-20 protein expressed in Esc herichia coli. The anti-F1-20 monoclonal antibody (mAb) stains a bovin e protein with an apparent molecular mass on SDS-PAGE of 190 kDa that copurifies with brain clathrin-coated vesicles (CCVs) and that can be extracted from the brain CCVs under conditions that extract AP-3. The anti-F1-20 and anti-AP-3 mAbs specifically recognize the same spot on a two-dimensional gel run on a bovine brain clathrin-coated vesicle ex tract. AP-3 purified from bovine brain CCVs is recognized by both the anti-F1-20 and anti-AP-3 mAbs. Purified preparations of bovine AP-3 an d bacterially, expressed mouse F1-20 give identical patterns of protea se digestion with bromelain and subtilisin. Sequence analyses reveal t hat F1-20 has an essentially neutral 30-kDa NH2-terminal domain with a n amino acid composition typical of a globular structure and an acidic COOH-terminal domain rich in proline, serine, threonine, and alanine. This is consistent with proteolysis experiments that suggested that A P-3 could be divided into a 30-kDa globular uncharged clathrin-binding domain and an acidic, anomalously migrating domain.