Evolution of Glycoprotein Hormone Subunit Genes in Bilateral Metazoa: Identification of Two Novel Human Glycoprotein Hormone Subunit Family Genes, GPA2 and GPB5

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Abstract
The canonical members of the human glycoprotein hormone subunit family of cystine knot-forming polypeptides include the common -subunit, and four -subunit genes, FSH ,L H , TSH, and hCG. Using pairwise sequence analysis of the complete human genome, we have identified two novel glycoprotein hormone subunit-related genes. Based on unique sequence similarity to the - and -subunits of glycoprotein hormones, they were named glycoprotein-2 (GPA2) and glycoprotein-5 (GPB5), respectively. PCR analysis using a panel of human cDNAs from 14 different tissues demon- strated that GPB5 is similar to other -subunits showing restricted tissue expression, mainly in pitu- itary and brain. In contrast, the GPA2 transcript is found in diverse tissues. Furthermore, immunoreac- tive GPA2 and GPB5 were detected in the anterior pituitary of mouse and frog, whereas the expression of GPA2 and GPB5 in transfected cells resulted in the secretion of recombinant polypeptides in condi- tioned medium. After GenBank searches in lower organisms, glycoprotein hormone -subunit-related genes were identified from the genome of nematode Caenorhabditis elegans, hookworm Ancylostoma caninum, and Drosophila melanogaster. The evolu- tionary conservation of these invertebrate homologs can be seen in several key sequence characteristics, and the data suggest that the glycoprotein hormone -subunit gene ancestor evolved before the emer- gence of bilateral metazoa, thus providing a better understanding of the evolution of this group of clas- sic polypeptide hormones and their receptors. Stud- ies of the complete inventory of genes homologous to glycoprotein hormone subunits in the human ge- nome and lower organisms will allow future func- tional characterization and identification of their re- spective receptors. (Molecular Endocrinology 16: 1538-1551, 2002)

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