Abstract
Prostacyclin, a potent vasodilator and inhibitor of platelet aggregation, acts through a cell-surface G protein-coupled receptor [prostacyclin (IP)]. The human (h) IP contains two consensus sites for N-linked glycosylation (N7 and N78). However, the role of glycosylation is unknown. Mutant receptors (N7-Q7,N78-Q78and N7,N78-Q7,Q78) were generated by replacing N7 and/or N78 with Q's. Receptor glycosylation was similar in the wild-type and N7-Q7 and was inhibited with tunicamycin. N78-Q78 and N7,N78-Q7,Q78demonstrated little or no glycosylation. Membrane localization was reduced for each mutant concomitant with impaired glycosylation. Partial localization to the plasma membrane allowed direct examination of the effect of glycosylation on IP function. High-affinity binding to N7-Q7 was similar(Kd = 21.7 ± 1.7 nM,n = 4) to that of the wild-type receptor (Kd = 24.3 ± 3.6 nM,n = 4), despite a reduced value forBmax (0.35 ± 0.03 fmol/mg of protein versus 3.34 ± 0.52 fmol/mg of protein, n = 4). Binding to N78-Q78(Bmax = 0.27 ± 0.03 fmol/mg of protein, n = 3;Kd = 149.1 ± 11.1,n = 3) and N7,N78-Q7,Q78 (no specific binding) was further impaired. Agonist-induced adenylyl cyclase activation was reduced in N7-Q7 cells, whereas N78-Q78 cells responded only to high concentrations of iloprost and N7,N78-Q7,Q78 were unresponsive. Inositol phosphate generation was evident only with the wild-type. Only the wild-type and N7-Q7receptors underwent agonist-induced sequestration. Our findings demonstrate greater glycosylation at N78 compared with N7. The extent of N-linked glycosylation of hIP may be important for membrane localization, ligand binding, and signal transduction.
Footnotes
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This work was supported by National Institute of Health Grants HL62250 and HL57847 and American Heart Association Grant 9906209U (to E.M.S).
- Abbreviations:
- PGI2
- prostacyclin
- GPCR
- G protein-coupled receptor
- hIP
- human prostacyclin receptor
- PLC
- phospholipase C
- TP
- thromboxane A2 receptor
- EP
- prostaglandin E2 receptor
- HEK
- human embryonic kidney
- GFP
- green fluorescent protein
- ELISA
- enzyme-linked immunosorbent assay
- HA
- hemagglutinin
- PBS
- phosphate-buffered saline
- FI
- fluorescent intensity
- Received March 13, 2001.
- Accepted June 8, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
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