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 Tyr89-His90-Lys91-Ser92), 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 Ser92 and alpha His90 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 Lys91 but alpha Ser92 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 Lys91 or alpha His90 with either a hydrophobic or a bulkier residues 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 His90 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 Ser92 in hTSH activity explains the evolutionary constraint to preserve the alpha-carboxy-terminal Ser92 in all glycoprotein hormones.
D000595:Amino Acid Sequence; D000818:Animals; D002455:Cell Division; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000096:biosynthesis; D006801:Humans; D008969:Molecular Sequence Data; D051381:Rats; D011989:Receptors, Thyrotropin / Q000502:physiology; D011994:Recombinant Proteins; D015398:Signal Transduction; D013329:Structure-Activity Relationship; D013972:Thyrotropin / Q000737:chemistry*; D014284:Triiodothyronine / Q000378:metabolism
D011989:Receptors, Thyrotropin; D011994:Recombinant Proteins; D014284:Triiodothyronine; D013972:Thyrotropin; D000242:Cyclic AMP