The role of the 3′ region of mammalian gonadotropin β subunit gene in the luteinizing hormone to chorionic gonadotropin evolution.

The research investigates why the chorionic gonadotropin (CG) hormone evolved only in certain mammals by studying the properties of a key protein sequence in the luteinizing hormone (LH), from which CG evolved. Results indicate that the absence of certain characteristics in the protein sequence of non-primate and non-equine mammals could explain why CG did not evolve in these species.

Understanding LH and CG Hormones

• The study revolves around two key hormones – luteinizing hormone (LH) and chorionic gonadotropin (CG). Both play crucial roles in the reproductive system with LH being present in all mammals, while CG evolved only in primates and equids.
• CG differentiates from LH by having an unique carboxyl-terminal peptide (CTP) at the end of its subunit. The CTP carries certain characteristics that enhance the functionality of CG, such as serum half-life extension.
• The research seeks to understand why the evolution of CG, which involves the integration of a previously untranslated region of the LH gene into the reading frame, did not occur in other mammals like ruminants.

Approach to the Research

• Researchers focused on studying the CTP from a bovine LH gene (boCTP) to see if it possessed key characteristics of the human CG subunit’s CTP (hCGβ-CTP).
• In-depth examination of boCTP was carried out to understand its properties and why it didn’t evolve into CG in ruminants.

Key Findings

• The boCTP didn’t inhibit crucial aspects of hormone synthesis. However, it lacked key elements for circulatory survival, most importantly, the O-linked glycans, sugar molecules that play an important role in protein structure and function.
• The absence of O-glycans in boCTP was identified as a major drawback as it could not extend serum half-life, a key determinant in evolution from LH to CG.
• The research suggests this deficiency in the properties of boCTP could be why ruminants developed alternative mechanisms to delay luteolysis (degradation of the corpus luteum) during early gestation, instead of evolving LH into CG.