ant sequence similarity among the different GPCR families favors this assumption. Other explanations for the lack of significant sequence similarities might be an extraordinary divergence or technical problems of the sequence-analysis methods used in analyzing polytopic membrane proteins or large protein 
families. Receptor family A subgroups In contrast to the subfamilies presented in GPCRDB, a database widely used in the field, our grouping shows the orphan receptors within their respective subgroup and their relationship to receptors with known ligands. In addition, our method sometimes resulted in subgroups with members whose ligands belong to different substance classes. These results are discussed in more detail below. Chemokine receptors Groups A1 and A2 comprise the chemokine receptors. The chemokine ligand superfamily is defined by four conserved cysteines that form two disulfide bonds, and can be structurally subdivided into two major branches based on the spacing of the first cysteine pair. Chemokines in which these residues are adjacent form the CC subfamily, and those separated by a single amino acid comprise the CXC subfamily. We had to divide the whole subfamily into two groups to perform a detailed phylogenetic analysis. This subgrouping produced the same dichotomy, as suggested by the two-ligand motifs, as another example of the parallel evolution of receptors and ligands. Similar results describing this parallel evolution were found previously using a different computational approach. Group A1 mainly comprises the CC family. We hypothesize that the orphan receptor CKRX, which constitutes a separate branch related to CKR1, 2, 3 and 5, might also bind a CC ligand. In contrast, TM7SF1 in this group seems to be only distantly, if at all, related to family-A receptors. It was grouped according to BLASTP results, where a misleading local alignment of approximately 20 amino acids placed it in the vicinity of the chemokine receptors. Group A2 is more heterogeneous and comprises receptors for CC and CXC ligands, as well as an orphan receptor previously thought to bind the peptide adrenomedullin. Adrenomedullin has now been shown to bind PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19816210 a family-B receptor and is discussed further below. The orphan receptor RDC1 in group A2 was first believed to be a receptor for vasointestinal peptide VIP, a notion not supported by phylogeny and later dismissed by experimental data. The opioid precursors preproenkephalin, preprodynorphin, prepro-opiomelanocortin and prepronociceptin display a strikingly similar general organization and a conserved amino-terminal region that contains six cysteines, probably involved in disulfide bond formation. The processed neuropeptides, in contrast, are less similar to each other. It could be speculated that the receptors first bound the precursors themselves, and that the diversity derived from processing is evolutionarily new. Processing prepronociceptin gives rise to two evolutionarily conserved peptides SCH58261 web besides orphanin FQ, the ligand for OPRX. It has not been reported whether these peptides bind to the orphan receptors GPR7 and GPR8, which constitute a new branch related to the opioid receptors. In group A5 we find three receptors that bind the 30-aminoacid peptide galanin, and related to these the GPR54 receptor, which is activated by the 54-, 14-, and 13-amino-acid peptides derived from the product of KiSS-1, a metastasis Peptide receptors Group A3 consists of receptors for the small peptides angioten