Molecular Endocrinology, Vol 9, 948-958, Copyright © 1995 by Endocrine Society

ARTICLES

Role of the carboxy-terminal residues of the alpha-subunit in the expression and bioactivity of human thyroid-stimulating hormone

M Grossmann, MW Szkudlinski, H Zeng, Z Kraiem, I Ji, JE Tropea, TH Ji and BD Weintraub
Molecular and Cellular Endocrinology Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-1758, USA.

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.

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