The Glycoprotein α-Subunit is Critical for Secretion and Stability of the Human Thyrotropin β-Subunit

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Abstract
TSH is a member of a family of heterodimeric glycoprotein hormones which have a common .alpha.-subunit but differ in their hormone-specific .beta.-subunit. To study the posttranslational processing and assembly of human TSH, eukaryotic expression vectors were constructed that contained either the human TSH.beta. gene only or both the TSH.beta. and .alpha.-genes. These vectors were transfected into Chinese hamster ovary cells and stable cell lines synthesizing TSH.beta. or TSH dimer were isolated. The kinetics of secretion of TSH.beta. and the rate of assembly of TSH dimer were compared to the known secretion and assembly of human LH and human CG. In the absence of the .alpha.-subunit, CG.beta. is secreted efficiently, but TSH and LH.beta.-subunits are slowly degraded intracellularly (t1/2 .apprxeq. 6 h) and less than 10% is secreted into the medium. In the presence of the .alpha.-subunit CG.beta. was also secreted efficiently as dimer but only 50% of the LH.beta. appeared in the medium as LH dimer. However, unlike LH.beta., the .alpha.-subunit efficiently combines with TSH.beta. since greater than 95% was secreted as TSH dimer. Thus, the determinants for human TSH.beta. secretion and assembly are unique from the other human glycoprotein hormone .beta.-subunits.
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