AP50 is the 50,000-dalton protein component found in clathrin-coated vesicles as part of the coat assembly protein (AP) complex, AP-2. AP50 cDNA clones were isolated from rat brain cDNA libraries, and their nucleotide sequence was determined. The isolated cDNA clones represent the entire coding sequence for the rat brain AP50. They encode a polypeptide containing 435 amino acids with a molecular weight of 49,612 daltons. Comparison with the partially sequenced bovine brain AP50 shows a primary structure that is highly conserved. AP50 does not have detectable sequence similarity with other known kinases or with other proteins of known sequence.