The carboxyl-terminal domain of the human pregnancy-specific glycoprotein specifies intracellular retention and stability
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1Human Genetics Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892.
Insights
The carboxyl-terminal (COOH) domains of pregnancy-specific glycoproteins (PSGs) influence their secretion. The hydrophobic COOH domain of PSG11w causes intracellular retention, while shorter domains promote secretion, impacting placental protein trafficking.
Area of Science:
- Biochemistry
- Cell Biology
- Reproductive Biology
Background:
- Pregnancy-specific glycoproteins (PSGs) are key placental proteins from the immunoglobulin superfamily.
- Over 30 PSG members exist, encoded by at least 11 genes, sharing high sequence homology but differing in COOH domains.
Purpose of the Study:
- To investigate the impact of PSG carboxyl-terminal (COOH) domains on protein secretion and stability.
- To determine how specific COOH domain structures dictate PSG intracellular trafficking and secretion.
Main Methods:
- Constructing chimeric PSGs with varying COOH domains (short hydrophilic, short hydrophobic, long hydrophobic).
- Analyzing protein secretion and cellular retention using Brefeldin A, endoglycosidase H sensitivity, and indirect immunofluorescence.
- Performing deletion analysis on the PSG11w COOH domain to identify critical regions for retention.
Main Results:
- Most PSGs are secretory, but PSG11w with a long hydrophobic COOH domain is retained intracellularly.
- Replacing the PSG11w COOH domain with a short one (PSG1e) resulted in secretion of the chimera.
- Chimeras with the PSG11w COOH domain were retained intracellularly, confirming its role in retention.
- Deletion analysis indicated that the first 12 residues of the PSG11w COOH domain are crucial for secretion, while longer hydrophobic segments cause retention.
- PSG11w and cell-associated mutants localize to and degrade within the endoplasmic reticulum.
Conclusions:
- The COOH domain of PSGs is a critical determinant of their secretion and intracellular fate.
- The hydrophobic nature and length of the COOH domain regulate PSG trafficking from the endoplasmic reticulum.
- Understanding PSG COOH domain function is vital for comprehending placental protein secretion and function during pregnancy.
Abstract:
The pregnancy-specific glycoproteins (PSGs), which are members of the immunoglobulin superfamily, are the major pregnancy-associated proteins synthesized by the human placenta. Thirty or more PSG members have been 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 study, we examined the effects of PSG COOH domains on protein secretion and stability. Using PSGs containing short (11-12 residues) hydrophilic (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 with a short COOH domain of PSG1e, the resulting PSG-N11w/C1e chimera became secreted into the medium. On the other hand, chimeras that harbored the PSG11w COOH domain, PSG-N1e/C11w and PSG-N16a/C11w, remained in cells, demonstrating that the COOH domain of PSG11w confers intracellular retention. Deletion analysis showed that mutant (PSG11w-C2) that contained the first 21 amino acids of PSG11w COOH domain or mutant (PSG11w-C3) that contained a deletion of hydrophobic residues 372-392 in the PSG11w COOH domain remained largely in cells. In contrast, the PSG11w-C1 mutant which contained the first 12 residues of the PSG11w COOH domain became a secretory protein. Studies of PSG synthesis and processing in the presence of Brefeldin A, a drug that impedes protein transport from endoplasmic reticulum to the Golgi system, showed that PSG11w 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.