Expression of two forms of carp gonadotropin α subunit in insect cells by recombinant baculovirus

There are two types of cDNA clones (designated α1 and α2) encoding the α subunit of carp gonadotropin. These two cDNAs are derived from different genes and encode proteins that differ by seven amino acid residues (three in the signal peptide and four in the mature polypeptide). Expression of these two cDNAs in insect cells by recombinant baculovirus revealed that the α1 subunit, after noncovalent association with the β subunit, has the same potency as the native α subunit purified from the pituitary. In contrast, the α2 subunit can associate with the β subunit, but only to form an inactive gonadotropin. Competition of the α2 subunit with the α1 subunit for association with the β subunit decreases the gonadotropin activity of the α/β complex. In addition, both α1 and α2 subunits are secreted into the culture medium by insect cells and have an apparent molecular mass approximately 5 kDa higher than that of the native α subunit. These results indicate that the insect cell-derived α1 subunit is biologically active and that those four amino acid changes in the mature α2 protein affect the biological activity and thus provide valuable clues for the study of the structure-function relationship of the α subunit of glycoprotein hormones.