The Molecular Biology of Avian Gonadotropin1

ABSTRACT Complimentary DNA for precursor molecules of chicken and quail luteinizing hormone (LH) /3 subunits have been cloned. From nucleotide sequences of the cDNA, the primary structures of the LH /3 subunit molecules of these avian species were deduced. The primary structures were similar, the amino acid sequence homology being 91.6%. For the a subunit, cloning of cDNA has been performed also in these two avian species. Predicted primary structures of the a subunit molecules of these birds were completely identical, although nucleotide sequences of their cDNA were slightly different. The LH /3 subunit molecules of both birds had 15 Pro residues at the same positions. Ten of them shared the same positions with Pro residues in the mammalian LH /3 subunits, in which about 20 Pro residues exist. Prediction of the secondary structure and exposure of each amino acid residue in the chicken LH 0 subunit molecule were performed with the aid of a computer program for protein engineering. It was revealed that most of 15 Pro residues did not exist in regions where the $ structure was predicted but were distributed in regions where a turn or loop was predicted. In addition, three of four Tyr residues were predicted to be located inside the molecule. These results suggest that the predicted presence of a number of Pro residues in the loop of the secondary structure is a cause of high animal group and hormone specificities in the LH-LH receptor interaction. The predicted internal localization of Tyr residues is considered to cause loss of receptor binding activity by conventional radioiodi-nation procedures. (Key words: luteinizing hormone, follicle-stimulating hormone, primary struc-ture, secondary structure, reproduction