Binding of clathrin assembly protein to clathrin triskelia induces their assembly into clathrin-coated vesicle in neurons. The clathrin binding protein is a neuronal-specific, synapse associated protein that is expressed nonuniformly in rat brain. We isolated two cDNAs, encoding the novel clathrin assembly protein, which has a 73% amino acid homology compared with that of AP180 protein when translated into amino acids. The deduced molecular weight is 64kD. The N-terminal domain harbouring clathrin binding site is very similar to that of AP180, and the C-terminal domain is much more different with that of AP180, which suggests that the novel protein mediates the assembly of clathrin and its regulatory role in the release of secretory vesicle.