THE ALTERNATIVELY SPLICED TYPE-II CORTICOTROPIN-RELEASING FACTOR-RECEPTOR, STABLY EXPRESSED IN LLCPK-1 CELLS, IS NOT WELL COUPLED TO THE G-PROTEIN(S)

Two alternatively spliced corticotropin-releasing factor receptor (CRF -R) cDNAs, type I and type II, were recently isolated from a human cDN A library. The two cDNAs are identical except that the type II cDNA en codes an additional 29 amino acid inserted in the first putative cytop lasmic loop. Since the first cytoplasmic loop is highly conserved in a ll the members of the hCRF receptor family we have examined whether th e presence of the 29 amino acid cassette in CRF-RII influences G prote in coupling in LLCPK-1 cells stably expressing the type I and type II hCRF receptors. Whether measured in intact cells or in membrane prepar ations, LLCPK-1 cells stably expressing CRF-RII have a 4-5-fold lower binding affinity. Maximal CRF-stimulated cAMP accumulation in LLCPK-1 cells stably expressing CRF-RT was 10-15-fold higher than that in LLCP K-1 cells expressing CRF-RII. The EC50 for CRF-stimulated cAMP accumul ation in hCRF-RI-expressing cells was in the range of 0.5 +/- 0.2 nM. In contrast, the EC50 for CRF-stimulated cAMP accumulation in hCRF-RII expressing cells was 7.7 +/- 0.2 nM. hCRF increased phosphoinositide turnover in LLCPK-1 cells stably expressing CRF-RI but not in those ex pressing CRF-RII; this effect required hCRF concentrations of 100 nM a nd higher. In membrane preparations, GTP-gamma-S inhibited hCRF bindin g to CRF-RII and shifted the binding Kd from 4.5 nM to 16.7 nM. Conver sely, GTP-gamma-S did not influence hCRF binding to CRF-RII in broken cell membranes. Additionally, CRF-stimulated adenylate cyclase activit y in cell membranes expressing CRF-RI was potentiated by GTP, whereas CRF-stimulated adenylate cyclase activity in cell membranes expressing CRF-RII was insensitive to GTP. These data indicate that CRF-RII is n ot well coupled to the G protein. Since the only difference between th e CRF-RII and CRF-RI is the insert in the first putative cytoplasmic l oop, these data indicate that the first cytoplasmic loop plays a cruci al role in hCRF receptor coupling to the G protein. (C) 1995 Academic Press, Inc.