The regulation of G protein activation by the rat corticotropin-releasing factor receptor type 1 (rCRFR1) in human embryonic kidney (HEK)293 (HEK-rCRFR1) cell membranes was studied. S]GTP␥S-bound G␣ q/11 revealed additional coupling to G q/11 , which also was homologously desensitized. Although G␣ q/11 coupling was PTX-insensitive, half of the sauvagine-stimulated accumulation of inositol phosphates in the cells was PTX-sensitive, suggesting involvement of G i in addition to G q/11 in the stimulation of inositol metabolism. It is concluded that CRFR1 signals through at least two different ways, one leading to G s -and G q/11 -mediated signaling steps and desensitization and another leading to G i -mediated signals without being desensitized. Furthermore, the concentrations of the stimulating ligand and GTP and desensitization may be part of a regulatory mechanism determining the actual ratio of the coupling of CRFR1 to different G proteins.The hypothalamic peptide corticotropin-releasing factor (CRF) 1 not only regulates the stress response in mammals by activation of the pituitary adrenal axis (1) but is also involved in the control of the immune response, cardiovascular, reproductive, and cognitive function, ingestive behavior, pregnancy and labor (for a review, see Refs. 2 and 3). The multiple actions of CRF are mediated by two classes of specific CRF receptors, CRFR1 (4 -6) and CRFR2 (7,8), which are encoded by unique genes and of which some variants exist, produced by alternative processing of the transcripts from each of the genes (for

Approximately 5-10% of the GPCRs (G-protein-coupled receptors) contain N-terminal signal peptides that are cleaved off during receptor insertion into the ER (endoplasmic reticulum) membrane by the signal peptidases of the ER. The reason as to why only a subset of GPCRs requires these additional signal peptides is not known. We have recently shown that the signal peptide of the human ET(B)-R (endothelin B receptor) does not influence receptor expression but is necessary for the translocation of the receptor's N-tail across the ER membrane and thus for the establishment of a functional receptor [Köchl, Alken, Rutz, Krause, Oksche, Rosenthal and Schülein (2002) J. Biol. Chem. 277, 16131-16138]. In the present study, we show that the signal peptide of the rat CRF-R1 (corticotropin-releasing factor receptor 1) has a different function: a mutant of the CRF-R1 lacking the signal peptide was functional and displayed wild-type properties with respect to ligand binding and activation of adenylate cyclase. However, immunoblot analysis and confocal laser scanning microscopy revealed that the mutant receptor was expressed at 10-fold lower levels than the wild-type receptor. Northern-blot and in vitro transcription translation analyses precluded the possibility that the reduced receptor expression is due to decreased transcription or translation levels. Thus the signal peptide of the CRF-R1 promotes an early step of receptor biogenesis, such as targeting of the nascent chain to the ER membrane and/or the gating of the protein-conducting translocon of the ER membrane.

Background and purpose: According to the two-domain model for the corticotropin-releasing factor receptor type 1 (CRF 1 ), peptide antagonists bind to the N-terminal domain (N-domain), non-peptide antagonists to the transmembrane region (J-domain), whereas peptide agonists attach to both the N-and J-domain of the receptor to express activity. The aim of this study was to search for possible differences in the antagonism of the Gs-and Gi-protein coupling of CRF 1 by a peptide (a-helical CRF(9-41)) and non-peptide antagonist (antalarmin), to determine whether the conformational requirements of the activated CRF 1 states for Gs and Gi coupling are similar or different. Experimental approach: We studied the inhibitory effect of a-helical CRF(9-41) and antalarmin on the coupling of CRF 1 to Gs-and Gi-protein in human embryonic kidney cells, using the [ 35 S]-GTPgS binding stimulation assay. Key results: The non-peptide antagonized the receptor coupling to Gs competitively but that to Gi noncompetitively, and its antagonistic potency was different for urocortin-and sauvagine-evoked G-protein activation. In contrast, the peptide antagonist exhibited uniformly competitive antagonism. Conclusions and Implications:The results allow us to extend the two-domain model of CRF 1 activation by assuming that CRF 1 agonists activate the receptor by binding to at least two ensembles of J-domain configurations which couple to Gs or Gi, that are in turn antagonized by a non-peptide antagonist competitively and allosterically, respectively. It is further concluded that the allosteric mechanism of non-peptide antagonism is not valid for the Gs-mediated physiological activities of CRF 1 .

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