Angelova K, Fremont V, Jain R, Zhang M, Puett D, Narayan P, Szkudlinski MW. Human alpha-subunit analogs act as partial agonists to the thyroid-stimulating hormone receptor: differential effects of free and yoked subunits.
Endocrine 2004;
24:25-31. [PMID:
15249700 DOI:
10.1385/endo:24:1:025]
[Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/10/2004] [Revised: 03/18/2004] [Accepted: 04/07/2004] [Indexed: 11/11/2022]
Abstract
The alpha-subunit is common to the heterodimeric glycoprotein hormones and has been highly conserved throughout vertebrate evolution. In an effort to determine if wild-type and engineered human alpha analogs can serve as agonists or antagonists to the human thyroid-stimulating hormone (TSH) receptor (TSHR), a potent alpha mutant, obtained by replacing four amino acid residues with lysine (alpha4K), was assayed and compared with the wild-type alpha-subunit. When added to CHO cells expressing TSHR, alpha4K, and to a very limited extent the fused homodimer, alpha4K-alpha4K, but not alpha, exhibited agonist activity as judged by cAMP production. When yoked to TSHR to yield fusion proteins, neither alpha, alpha4K, alpha-alpha, nor alpha4K-alpha4K activated TSHR, although yoked alpha4K and alpha4K-alpha4K were weak inhibitors of TSH binding to TSHR. The yoked subunit-receptor complexes were, however, functional as evidenced by increased cAMP production in cells co-expressing human TSHbeta and alpha-TSHR, alpha4K-TSHR, alpha-alpha-TSHR, and alpha4K-alpha4K-TSHR. These results demonstrate that agonists to TSHR can be obtained with alpha-subunit analogs and suggest that rational protein engineering may lead to more potent alpha-based derivatives. The differences found between the experimental paradigms of adding free alpha analogs to TSHR and covalent attachment are attributed to con-formational constraints imposed by fusion of the alpha-subunit analog and receptor, and may suggest an important role for a free (C-terminal) alpha-carboxyl in the absence of the beta-subunit.
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