We have cloned the cDNA for human carboxypeptidase D (CPD), a new B-type metallocarboxypeptidase that is membrane bound and has an acidic pH optimum. The 5.8 kb of cDNA sequenced contains an open reading frame of 4131 bp encoding 1377 amino acid residues. The sequence is similar (75% identity) to duck gp180, a protein that was isolated, cloned and sequenced as a hepatitis B virus-binding protein but not characterized as a carboxypeptidase. Hydropathic analysis revealed a hydrophobic region at the N-terminus, representing the signal peptide, and one near the C-terminus that probably represents the transmembrane anchor. The most striking feature is the presence of three tandem carboxypeptidase homology domains that have sequence similarity to the regulatory B-type carboxypeptidase family, typified by carboxypeptidases M, E and N. Because of the three repeats, CPD is about three times larger (175–180 kDa) than other members of this family (approx. 50–62 kDa). Domain 2 is most closely related to carboxypeptidases M, E and N (45–48% identity), followed by domain 1 (37–38%) and domain 3 (20–27%). There is much higher sequence identity in regions containing putative active site residues, and all catalytically important residues are strictly conserved in domains 1 and 2. In domain 3, however, only 1 of 8 active site residues is conserved, indicating that this portion might not be catalytically active. Northern blotting of mRNA from human tissues and cells showed high levels of CPD mRNA in placenta, pancreas and Hep G2 hepatoma cells, and smaller amounts in skeletal muscle, heart and HT-29 colon carcinoma and melanoma cell lines.

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