Re-evaluation of chicken CXCR1 determines the true gene structure: CXCLi1 (K60) and CXCLi2 (CAF/interleukin-8) are ligands for this receptor

The original report of chicken CXCR1 (Li, Q. J., Lu, S., Ye, R. D., and Martins-Green, M. (2000) Gene (Amst.) 257, 307-317) described it as a single exon gene, with two isoforms (differing in their start codon). In comparison with mammalian CXCR1, the reported chicken CXCR1 was longer at both the NH(2) and COOH termini, and it lacked the conserved (C/S)CXNP motif present in the last transmembrane region of all known chemokine receptors. A re-evaluation of chicken CXCR1, comparing known expressed sequence tags with the chicken genome sequence, suggested that the gene contains two exons. We isolated a cDNA corresponding to our prediction, which was significantly different in sequence to the reported CXCR1. In particular, there were three frameshifts in our sequence, compared with the reported sequence, that restored higher identity in the COOH-terminal half of the protein to mammalian CXCR1 (61% total amino acid identity compared with 52% for the reported CXCR1), restored the (C/S)CXNP motif, and gave a predicted protein of the same length as mammalian CXCR1. In human, CXCR1 is the receptor for CXCL8. In the chicken, there are two syntenic genes, CXCLi1 and CXCLi2, which look equally like orthologues of human CXCL8. We demonstrate that both of these chemokines are ligands for chicken CXCR1. We also demonstrate that heterophils express chicken CXCR1 and that the receptor is Galpha(i) protein-linked.