Class I receptors have been predicted to have the N-terminal in the interior of the cell while Class II receptors have the usual GPCR topology of the N-terminal outside of the cell and the C-terminal inside the cell [8, 20]. Due to the predicted membrane topology of the progesterone receptors, it is suggests that they might be a new class of GPCRs. In this paper we report a new member of the Class II PAQRs and address the issues regarding membrane topology, ligand binding and its relationship to the S. schenckii G alpha subunit SSG-2, in an effort to characterize the SsPAQR1. The fact that SsPAQR1 was

identified in a Y2H assay with a G protein alpha subunit as bait, offers for the first

time direct see more evidence of the association of these receptors to the heterotrimeric G protein signalling pathways. This association was verified using Co-IP. Indirect evidence of the association of progesterone PAQRs to G proteins has been reported by other investigators. beta-catenin inhibitor One of these instances involves fish oocyte maturation where response to a novel progesterone hormone was associated to a pertussis-sensitive Gαi subunit pathway [6, 11, 40]. Transmembrane analysis of the SsPAQR1 described here predicts that this protein has the 7 transmembrane domains characteristic of GPCRs like other progesterone binding members of the PAQR family. The bioinformatic analyses described above (TMHMM, SOUSI and MEMSAT-SVM) predicted that the N-terminal region is localized outside the plasma membrane while the C-terminal region

is intracellular. This orientation has also been observed in progestin receptors, PAQR6 and mPRa [6]. In the case of the adiponectin members of the PAQR family such as the human adiponectin receptor 2 and 3, the orientation seems to be the opposite, as stated previously [12, 41]. Bioinformatic analyses also show that SsPAQR1 Sclareol and its fungal homologues from M. oryzae, T. reesei, N. crassa and P. anserina, among others belong to the PAQR receptor family. These homologues exhibit approximately 65 to 80% identity to SsPAQR1. The transmembrane domain analyses of some of these fungal homologues showed that most have the 7 transmembrane domains characteristic of the GPCRs. TMHMM analysis also shows that they have the traditional orientation of an external N-terminal domain and an internal C-terminal domain as SsPAQR1, except in the case of Izh3 where the N-terminal is inside and the C-terminal is outside (Additional file 2).