The LHβ 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

The expression of a previously untranslated carboxyl-terminal sequence is associated with the ancestral lutropin (LH) β to the β-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 LHβ 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 LHβ 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 LHβ gene (boCTP) would be sufficient to generate the LHβ species of a ruminant with properties typical to the CGβ subunit. The mutated bovine LHβ-boCTP subunit was expressed and N-glycosylated in transfected Chinese hamster ovary cells. However, unlike human (h) CGβ 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 CGβ subunit. Moreover, when expressed in polarized Madin-Darby canine kidney cells, this hCGβ-boCTP chimera was secreted basolaterally rather than from the apical compartment, which is the route of the wild type hCGβ 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 LHβ to CGβ evolution did not occur in ruminants. We propose that in primates and equids, further natural mutations in the progenitor LHβ gene resulted in the efficient O-glycosylation of the CTP, thus favoring the retention of an elongated reading frame.