Structure and Function of Novel Carboxypeptidase gp180

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Carboxypeptidase gp180 is a membrane-associated protein initially identified by us in duck hepatocytes on the basis of its ability to bind DHBV envelope proteins. However, we do not know the normal biological function of this enzyme in the uninfected host. To understand the nature of gp180, we have cloned human and mouse gp180 cDNAs and compared their amino acid sequences.Duck gp180 is about 3 times the size of carboxypeptidase H,but homologies to the basic carboxypeptidases can be found throughout the protein, in a fashion that indicates that gp180 is actually a head-to-tail array of carboxypeptidase homology domains.gp180 DNAs from human and mouse encode a protein of 1,380 and 1,377 amino acids, respectively. Similar to duck gp180, both human and mouse gp180 array three carboxypeptidase domains. The overall identity of human gp180 is 91% with mouse gp180, and that of mouse gp180 is 77% with duck gp180. The identities of domains A,B and C between these species are similar. Both domains A and B of them have conserved the residues known to reside at the carboxypeptidase catalytic center as well as the residues involved in zinc and substrate binding. On the other hand, domain C of them lacks these conserved residues. Domain C may be lost carboxypeptidase activity. These suggest that the gene for gp180 may have evolved by tandem duplication of ancestral carboxypeptidase coding sequence before the evolution of these species. We note that cytoplasmic domain of human gp180 displays amino acid sequence identities of 100 and 95% with mouse and duck gp180, respectively. This highly conserved cytoplasmic domain may play an important role in the function and/or cellular localization of gp180.