IDENTIFICATION OF A MEMBRANE-BOUND CARBOXYPEPTIDASE AS THE MAMMALIAN HOMOLOG OF DUCK GP180, A HEPATITIS-B VIRUS-BINDING PROTEIN

A unique membrane-bound carboxypeptidase was discovered and characteri zed in membrane fractions of human skin fibroblasts and the mouse mono cyte-macrophage cell line J774A. 1 and was partially purified from hum an placenta. Enzymatic characterization identified it as a new member of the regulatory B-type metallocarboxypeptidases, different from carb oxypeptidases B, E, M, N and U. It is, however, similar to the newly d escribed bovine carboxypeptidase D, suggested to be a homolog of duck gp180, a 180 kDa hepatitis B virus-binding protein. To prove this, a p artial cDNA encoding a 20 kDa fragment of the human homolog of duck gp 180 was expressed in bacteria and the recombinant protein was purified . Antibodies raised to the protein immunoprecipitated 94% or 72% of th e low pH carboxypeptidase activity in human skin fibroblasts or J774A. 1 cells and gave a 175 kDa protein band in Western blots. Thus, carbo xypeptidase D is the mammalian homolog of duck gp180 and is distribute d in a variety of different cell types.