Cloning, expression, and mutational analysis of the pigeon prolactin receptor

A full-length PRL receptor (PRLR) complementary DNA from pigeons was obtained by screening pigeon crop sac libraries and by reverse transcription coupled with polymerase chain reaction. The deduced sequence of 830 amino acids revealed a new member of the PRL/GH/cytokine receptor superfamily; it contained a single transmembrane domain, all of the conserved cysteine pairs and the WSxWS motif in its extracellular domain, and a conserved proline-rich motif in its intracellular domain. The cytoplasmic domain of the pigeon PRLR was similar to the long form of mammalian PRLRs in both length and general sequence characteristics. There was no evidence of short or intermediate form receptor in any of the clones examined. Unlike mammalian PRLRs, which have a single extracellular domain, the pigeon PRLR had two highly homologous units in its extracellular domain. Amino acid sequences of the repeated units were 64% identical to each other and were 52-59% (membrane-distal unit) and 60-71% (membrane-proximal unit) identical to the extracellular domain of the mammalian PRLRs. To study the significance of the repeated extracellular units in pigeons, a mutated pigeon PRLR complementary DNA that contained only the membrane-proximal unit in the extracellular domain was constructed. Both the wild-type and mutated pigeon PRLRs bound to rat PRL (rPRL) with equally high affinities (Ka = 0.6 nM-1). The ligand specificities of both forms of the pigeon PRLR were also identical.