Gonadotropin α-subunit glycoprotein from channel catfish (Ictalurus punctatus) and its expression during hormoneinduced ovulation

Complementary DNA (cDNA) encoding the channel catfish (Ictalurus punctatus) gonadotropin (GTH) α-subunit glycoprotein was cloned by polymerase chain reaction (PCR) from a plasmid library made from pituitary RNA. Complete cDNA cloning was achieved by carrying out two PCR reactions: one with an upstream sense primer plus the universal sequencing primer, located downstream of the poly(A) sequence of the cDNA in the plasmid vector, to amplify the downstream portion of the cDNA; the other with a downstream antisense primer plus the reverse-sequencing primer, located upstream of the very 5' end of the cDNA sense strand in the plasmid vector, to amplify the upstream portion of the cDNA. The two amplified fragments overlapped about 70 bp. Nucleotide sequence analysis revealed that the catfish GTH α-subunit was 658 bp encoding 116 amino acids and harboring a 5' nontranslated region (NTR) of 42 bp and a 3' NTR of 265 bp. The deduced amino acid sequence of the catfish GTH α-subunit is highly conserved with those from other cloned teleost GTH α-subunits. The GTH α-subunit was highly expressed even before induction for ovulation in females during spawning season. Administration of carp pituitary extract (a spawning-inducing reagent) induced only 1.4-fold higher expression of the GTH α-subunit RNA, but induced very rapid egg maturation and ovulation. This unexpected result indicated that the GTH α-subunit may not be the limiting factor for ovulation and spawning, which may be regulated by a change of proportional coupling of the GTH α-subunit with specific β-subunits during hormone-induced ovulation.