Glycoprotein Hormones: Structure Function Studies

Glycoprotein hormones, Thyrotropin(TSH), Follitropin(FSH), Lutropin(LH) and Chorionic Gonadotropin(CG) are heterodimers composed of a common \alpha subunit, non covalently associated with hormone specific \beta subunit. They are interesting models to study protein folding, protein-protein interaction and role of carbohydrates in protein structure and function. The studies using site directed mutagenesis are often hampered by the loss of heterodimerization. This limitation can be overcome by translationally fusing the subunits to obtain a single chain hormone. Single chain hCG , thus produced in the laboratory, has been shown to be structurally and functionally similar to native hCG. This fusion protein approach was used to study a mutation that disrupts heterodimerization, where a Proline38(P38) residue in \alpha was substituted by Alanine. The mutant was expressed using Pichia expression system. It was observed that though the overall conformation of the mutant was altered, several epitopes in the subunits were maintained as tested by RIA. The mutant retained its receptor binding ability but was biologically inactive. It can be inferred that the fusion protein strategy has successfully overcome the constraint of heterodimerization imposed by the mutation. P38 residue appears to play an important role in attaining a conformation that is optimal for bioactivity