The LHbeta gene of several mammals embeds a carboxyl-terminal peptide-like sequence revealing a critical role for mucin oligosaccharides in the evolution of lutropin to chorionic gonadotropin in the animal phyla

• Published in: The Journal of biological chemistry 2005-04, Vol.280 (17), p.16676-16684
• Main Authors: Nakav, Sigal, Jablonka-Shariff, Albina, Kaner, Shelly, Chadna-Mohanty, Prabhjit, Grotjan, H Edward, Ben-Menahem, David
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Base Sequence; Cattle; CHO Cells; Chorionic Gonadotropin - chemistry; Codon, Terminator; Computational Biology - methods; Cricetinae; DNA - metabolism; Dogs; Glycosylation; Humans; Immunoprecipitation; Luteinizing Hormone, beta Subunit - chemistry; Luteinizing Hormone, beta Subunit - physiology; Molecular Sequence Data; Mucins - chemistry; Mutation; Oligosaccharides - chemistry; Open Reading Frames; Peptides - chemistry; Placenta - metabolism; Polysaccharides - chemistry; Protein Structure, Tertiary; Recombinant Fusion Proteins - chemistry; Sequence Homology, Amino Acid; Time Factors; Transfection
• ISSN: 0021-9258

The expression of a previously untranslated carboxylterminal sequence is associated with the ancestral lutropin (LH) beta to the beta-subunit gene evolution of choriogonadotropins (CG). The peptide extension (denoted as CTP) is rich in mucin-type O-glycans and confers new hormonal properties on CG relative to the LH. Although the LHbeta gene is conserved among mammals and only a few frameshift mutations account for the extension, it is merely seen in primates and equids. Bioinformatics identified a CTP-like sequence that is encrypted in the LHbeta gene of several mammalian species but not in birds, amphibians, or fish. We then examined whether or not decoding of the cryptic CTP in the bovine LHbeta gene (boCTP) would be sufficient to generate the LHbeta species of a ruminant with properties typical to the CGbeta subunit. The mutated bovine LHbeta-boCTP subunit was expressed and N-glycosylated in transfected Chinese hamster ovary cells. However, unlike human (h) CGbeta CTP, the cryptic boCTP was devoid of mucin O-glycans. This deficiency was further confirmed when the boCTP domain was substituted for the natural CTP in the human CGbeta subunit. Moreover, when expressed in polarized Madin-Darby canine kidney cells, this hCGbeta-boCTP chimera was secreted basolaterally rather than from the apical compartment, which is the route of the wild type hCGbeta subunit, a sorting function attributed to the O-glycans attached to the CTP. This result shows that the cryptic peptide does not orientate CG to the apical face of the placenta, to the maternal circulation as seen in primates. The absence of this function, which distinguishes CG from LH, provides an explanation as to why the LHbeta to CGbeta evolution did not occur in ruminants. We propose that in primates and equids, further natural mutations in the progenitor LHbeta gene resulted in the efficient O-glycosylation of the CTP, thus favoring the retention of an elongated reading frame.