The G Protein–Coupled Receptor Subset of the Chicken Genome

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Abstract
G protein–coupled receptors (GPCRs) are one of the largest families of proteins, and here we scan the recently sequenced chicken genome for GPCRs. We use a homology-based approach, utilizing comparisons with all human GPCRs, to detect and verify chicken GPCRs from translated genomic alignments and Genscan predictions. We present 557 manually curated sequences for GPCRs from the chicken genome, of which 455 were previously not annotated. More than 60% of the chicken Genscan gene predictions with a human ortholog needed curation, which drastically changed the average percentage identity between the human–chicken orthologous pairs (from 56.3% to 72.9%). Of the non-olfactory chicken GPCRs, 79% had a one-to-one orthologous relationship to a human GPCR. The Frizzled, Secretin, and subgroups of the Rhodopsin families have high proportions of orthologous pairs, although the percentage of amino acid identity varies. Other groups show large differences, such as the Adhesion family and GPCRs that bind exogenous ligands. The chicken has only three bitter Taste 2 receptors, and it also lacks an ortholog to human TAS1R2 (one of three GPCRs in the human genome in the Taste 1 receptor family [TAS1R]), implying that the chicken's ability and mode of detecting both bitter and sweet taste may differ from the human's. The chicken genome contains at least 229 olfactory receptors, and the majority of these (218) originate from a chicken-specific expansion. To our knowledge, this dataset of chicken GPCRs is the largest curated dataset from a single gene family from a non-mammalian vertebrate. Both the updated human GPCR dataset, as well the chicken GPCR dataset, are available for download. Man and chicken are very different, but how is that difference related to our respective gene repertoire? The authors studied the family of G protein–coupled receptors (GPCRs), which in man contains about 791 proteins. These are found in the cell membrane, where they recognize substances, thereby functioning as mediators of signals across the cellular membrane. GPCRs respond to physiologically important substances such as hormones and neurotransmitters. In this paper, the publicly available genomic sequence from the domestic chicken is used to identify the entire repertoire of GPCRs in this species. The authors found 557 GPCRs and compared the chicken and human receptors; they concluded that out of the 328 chicken receptors that are not involved in olfaction, more than 250 have a corresponding human receptor. The majority of the differences between the chicken and man are within three groups of GPCRs—the receptors for olfaction, bitter taste, and the receptors involved in the immune system. The chicken GPCR sequences obtained here will be useful for identification of GPCRs in other species that are more distantly related to man, such as fish or insects. The domestic chicken represents the leading experimental model among the avian species and also serves as an important source of food worldwide.