THE CARBOXYL-TERMINAL DOMAIN OF THE HUMAN PREGNANCY-SPECIFIC GLYCOPROTEIN SPECIFIES INTRACELLULAR RETENTION AND STABILITY

The pregnancy-specific glycoproteins (PSGs), which are members of the immunoglobulin superfamily, are the major pregnancy-associated protein s synthesized by the human placenta. Thirty or more PSG members have b een identified which are encoded by at least 11 linked genes. The PSG proteins share 85-95% sequence homology in the coding region, but show variability at the carboxyl-terminal (COOH) domains. In the present s tudy, we examined the effects of PSG COOH domains on protein secretion and stability. Using PSGs containing short (11-12 residues) hydrophil ic (PSG1e, PSG11s, and PSG16a), short (22 residues) hydrophobic (PSG6r ), and long (81 residues) hydrophobic (PSG11w) COOH domains, we showed that most PSG members were secretory proteins except PSG11w which was largely retained in cells. When the PSG11w COOH domain was replaced w ith a short COOH domain of PSG1e, the resulting PSG-N11w/C1e chimera b ecame secreted into the medium. On the other hand, chimeras that harbo red the PSG11w COOH domain, PSG-N1e/C11w and PSG-N16a/C11w, remained i n cells, demonstrating that the COOH domain of PSG11w confers intracel lular retention. Deletion analysis showed that mutant (PSG11w-C2) that contained the first 21 amino acids of PSG11w COOH domain or mutant (P SG11w-C3) that contained a deletion of hydrophobic residues 372-392 in the PSG11w COOH domain remained largely in cells. In contrast, the PS G11w-C1 mutant which contained the first 12 residues of the PSG11w COO H domain became a secretory protein. Studies of PSG synthesis and proc essing in the presence of Brefeldin A, a drug that impedes protein tra nsport from endoplasmic reticulum to the Golgi system, showed that PSG 11w resided and degraded in the endoplasmic reticulum. The endoplasmic reticulum localization of PSG11w and the cell-associated mutant PSGs was further demonstrated by their sensitivity to endoglycosidase H and indirect immunofluorescence analysis.