Identification of ligands for orphan G-protein-coupled receptors by using human immunodeficiency virus

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(1)We have shown four G-protein-coupled receptors, GPR1,RDC1,D6 and FML1, function as co-receptors for HIV-1 infection. Among them, RDC1 and GPR1 are known to be orphan receptors, namely, their ligands have not been identified yet. The amino acid sequence of GPR1 shows little similarities to those of chemokine receptors. Therefore, to infer its ligand, we made a phylogenetic tree of GPCRs, which have been thought to be consisting of several hundred different types, using their similarities in their amino acid sequences deduced from their nucleotide sequences determined upon the Human Genome Projects. Chemokine receptors, consisting of receptors for CCR or CXCR chemokines, formed a single branch of the tree. GPR1 was the most similar to another orphan chemokine receptor, DEZ, and was also markedly similar to FML1. FML1 has been reported to be a receptor for oligoneptides derived from digested products of bacterial origins. Therefore, there is a possibility that GPR1 is also functioning as a receptor for exogenous oligoneptides.(2)We have shown that the peptides derived from apelin inhibit infection of HIV-1 tpNP-2 cell expressing APJ, a receptor for apelin. Next we tested whether it is possible to screen peptides for their probabilities as a ligand for orphan GPCRs, especially GPR1 and RDC1. The N-terminal peptides of GPR1, CCR5 or CXCR4 were synthesized and examined for abilities to inhibit HIV-1 infection to NP-2 cells expressing GPR1,CCR5 or CXCR4 as a co-receptor. Only the N-terminal peptide of GPR1 but CCR5 or CXR4, consisting of 27 amino acids, inhibited infection of NP-2 cells expressing not only GPR1 but also CCR5 or CXCR4 with HIV-1,HIV-2 or SIV. The NP-2/CD4 cell system to be infected with HIV-1 or HIV-2 will be suitable for screening of ligands for orphan GPCRs.(3)We have screened more than 100 oligopeptides obtained from the Protein Research Institute, Osaka. Unfortunately, none of the peptides markedly inhibited HIV-1 or HIV-2 infection.