CONVERSION OF THYROTROPIN HETERODIMER TO A BIOLOGICALLY-ACTIVE SINGLE-CHAIN

TSH and the gonadotropins, FSH, LH, and CG are a family of heterodimer ic glycoprotein hormones composed of a common a-subunit noncovalently linked to a hormone specific beta-subunit. Assembly of alpha- and beta -subunits is essential for hormone-specific posttranslational modifica tions, receptor binding, and bioactivity. Structure-function studies o f TSH and gonadotropins using site-directed mutagenesis can often affe ct folding, assembly, and secretion of the hormone. To circumvent thes e difficulties, recently, the gonadotropin heterodimers were converted to single chains. Here we converted the hTSH heterodimer to a biologi cally active single chain by genetically fusing the amino terminal end of the common alpha-subunit to the carboxyl terminal end of hTSH beta in the presence or absence of hCG beta carboxyl terminal peptide (CTP ), which was used as a linker. Wild-type hTSH and the single chains we re expressed in Chinese hamster ovary (CHO) cells, and they were effic iently secreted. Although the secretion rate of the single chain was 3 -fold higher than that of hTSH wild-type. Moreover, the secretion of t he single chain in the presence of the CTP linker was dramatically inc reased. On the other hand, receptor binding and in vitro bioactivity o f the single chains were similar to that of hTSH wild-type. These data indicate the potential of the single chain approach to further invest igate structure-function relationships of TSH.