Structure and Function of Novel Carboxypeptidase gp180

羧肽酶gp 180是一种膜相关蛋白，最初由我们在鸭肝细胞中根据其结合DHBV包膜蛋白的能力鉴定。然而，我们不知道这种酶在未感染宿主中的正常生物学功能。为了了解gp 180的性质，我们克隆了人和小鼠gp 180的cDNA，并比较了它们的氨基酸序列。鸭gp 180的大小约为羧肽酶H的3倍，但在整个蛋白质中可以发现与碱性羧肽酶的同源性，这表明GP 180实际上是一种头对羧肽酶同源结构域尾部阵列。来自人和小鼠的gp 180 DNA分别编码1，380和1，377个氨基酸的蛋白质。与鸭gp 180相似，人和小鼠gp 180都排列有三个羧肽酶结构域。人gp 180与小鼠gp 180的总体同一性为91%，小鼠gp 180与鸭gp 180的总体同一性为77%。这些物种之间的结构域A、B和C的同一性是相似的。它们的结构域A和B都保守了已知位于羧肽酶催化中心的残基以及参与锌和底物结合的残基。另一方面，它们的结构域C缺少这些保守残基。结构域C可能失去羧肽酶活性。这些表明，gp 180基因可能是在这些物种进化之前通过祖先羧肽酶编码序列的串联复制而进化的。我们注意到，人gp 180的胞质结构域显示与小鼠和鸭gp 180的氨基酸序列分别为100%和95%的同一性。这种高度保守的胞质结构域可能在gp 180的功能和/或细胞定位中起重要作用。

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.