The function of the common subunit of glycoprotein hormones

Molecular cloning and sequence analysis of chum salmon gonadotropin cDNAs

cDNAs encoding alpha and beta subunits of salmon gonadotropins, sGTHI and sGTHII, have been isolated from the cDNA library prepared from salmon pituitary mRNA. sGTHI alpha, sGTHI beta, and sGTHII beta cDNAs encode polypeptides of 114, 137, and 142 amino acids, including signal peptides of 22, 24, and 23 amino acids, respectively. The deduced amino acid sequence for sGTHI alpha revealed rather high homology (66-69%) to mammalian alpha chains, whereas sGTHI beta and sGTHII beta show lower homology (30%) to each other and to mammalian beta subunits. The existence of two distinct beta-subunit cDNAs in the teleost suggests that divergence of the GTH gene took place earlier than divergence of teleosts from the main line of evolution leading to tetrapods.

Nucleotide sequence of the mRNA encoding the pre-alpha-subunit of mouse thyrotropin

We have constructed and cloned in bacteria recombinant DNA molecules containing DNA sequences coding for the precursor of the alpha subunit of thyrotropin (pre-TSH-alpha). Double-stranded DNA complementary to total poly(A)+RNA derived from a mouse pituitary thyrotropic tumor was prepared enzymatically, inserted into the Pst I site of the plasmid pBR322 by using poly(dC).poly(dG) homopolymeric extensions, and cloned in Escherichia coli chi 1776. Cloned cDNAs encoding pre-TSH-alpha were identified by their hybridization to pre-TSH-alpha mRNA as determined by cell-free translations of hybrid-selected and hybrid-arrested RNA. The nucleotide sequences of two cDNAs (510 and 480 base pairs) were determined with chemical methods and corresponded to much of the region coding for the alpha subunit and the 3' untranslated region of pre-TSH-alpha mRNA. The sequence of the 5' end of the mRNA was determined from cDNA synthesized by using total mRNA as template and a restriction enzyme DNA fragment as primer. Together these sequences represented greater than 90% of the coding and noncoding regions of full-length pre-TSH-alpha mRNA, which was determined to be 800 bases long. The amino acid sequence of the pre-TSH-alpha deduced from the nucleotide sequence showed a NH2-terminal leader sequence of 24 amino acids followed by the 96-amino-acid sequence of the apoprotein of TSH-alpha. There is greater than 90% homology in the amino acid sequences among the murine, ruminant, and porcine alpha subunits and 75-80% homology among the murine, equine, and human alpha subunits. Several regions of the sequence remain absolutely conserved among all species, suggesting that these particular regions are essential for the biological function of the subunit. The successful cloning of the alpha subunit of TSH will permit further studies of the organization of the genes coding for the glycoprotein hormone subunits and the regulation of their expression.

Purification and receptor binding properties of complexes between lutropin and monovalent antibodies against its alpha subunit

A complex between bovine lutropin (LH) and monovalent antibodies (Fab fragments) directed against its alpha subunit, which is common to the glycoprotein hormones, has been purified by gel filtration and chromatography on concanavalin A-Sepharose. The complex is heterogenous with respect to molecular size; 70--80% of the hormone is complexed with either two or three Fab fragments. The LH-Fab alpha complexes retain only about 13% receptor binding activity as compared to LH when measured in a radioligand receptor assay in which the radiolabeled ligand is human choriogonadotropin. (Use of the human hormone as labeled ligand permits direct measurement of competition between receptor and the bovine complex because the alpha portion of the human hormone does not cross react significantly with antibodies directed against bovine alpha subunits.) Complex formation does not lead to dissociation of the lutropin into its subunits, as shown with a homologous LH-beta immunoassay which distinguishes free beta subunit from intact LH. Complexing of LH with Fab-alpha fragments also causes little or no change in the affinity of the hormone's beta subunit for anti-LH-beta antibodies indicating that significant changes in beta subunit conformation did not occur. The data show that at least two well-separated antigenic regions on the alpha subunit are exposed to the surface in the intact hormone. They are also in agreement with the proposal that the loss of binding activity to receptor is due to steric effects rather than to changes in conformation or dissociation, and that there may be sites on the alpha subunit which interact directly with the receptor.