ROLE OF THE CARBOXY-TERMINAL RESIDUES OF THE ALPHA-SUBUNIT IN THE EXPRESSION AND BIOACTIVITY OF HUMAN THYROID-STIMULATING HORMONE

The glycoprotein hormones TSH, CG, LH, and FSH are heterodimers consisting of a hormone-specific beta-subunit and a common alpha-subunit. The aim of the present study was to investigate the role of the carboxy terminus of the common alpha-subunit (amino acids Tyr(89)-His(90)-Lys(91)-Ser(92))(i) which has been shown to be important for human (h) CG and hFSH, for the activity of hTSH. Successive truncations of the alpha-carboxy terminus by site-directed mutagenesis revealed a stepwise reduction of bioactivity occurring at residues alpha Ser(92) and alpha His(90) to 64% and 13%, respectively. This contrasts with previous findings for hCG and hFSH, where loss of bioactivity occurred in a single step with the deletion of alpha Lys(91) but alpha Ser(92) was not. important. The decreased bioactivities of the hTSH alpha-truncation mutants were reflected by concomitant reductions of cAMP production, thyroid hormone synthesis and cell growth and were accompanied by a loss of receptor binding. Substitution of residues alpha Lys(91) or alpha His(90) with either a hydrophobic or a bulkier residue resulted in a reduction of receptor binding and signal transduction, indicating that the alpha-carboxy terminus of hTSH may interact with the TSH receptor in a tight contact area. Conversely, substitution of alpha His(90) with smaller residues enhanced bioactivity. In addition, the integrity of the alpha-carboxy terminus was essential for hTSH expression. Thus, we showed common and different roles of the alpha-carboxy-terminal residues for the glycoprotein hormones. The unique role of alpha Ser(92) in hTSH activity explains the evolutionary constraint to preserve the alpha-carboxy-terminal Ser(92) in all glycoprotein hormones.