R. D. Howells

Photoaffinity labelling of the TSH receptor on FRTL5, cells

An investigation of the properties of TSH receptors on FRTL5 cells using affinity labelling with a 125 I‐labelled photoactive derivative of TSH is described. Our studies suggest that FRTL5 cells contain 2 principal types of cell surface TSH receptors. One form, probably a precursor, consists of a single polypeptide chain (M r 120 000) with an intrachain loop of amino acids formed by a disulphide bridge. The other type of receptor consists of a water‐soluble A chain (M r 55000) linked to an amphiphilic B chain (M r 35000) by a disulphide bridge. The 2 chain structure is probably derived from the single chain 120 000 protein by enzymatic cleavage of peptide sequences within the loop of amino acids formed by the intrachain disulphide bridge.

A structure for the porcine TSH receptor

Affinity purified, detergent‐solubilised porcine TSH receptors have been crosslinked to a 125I‐labelled photoactive derivative of TSH and analysed by gel electrophoresis, gel filtration and sucrose density gradient centrifugation. Our studies suggest that the porcine TSH receptor is made up of a hydrophilic A subunit with an M r about 45000 linked to an amphiphilic B subunit (M rapprox. 25 000) by a disulphide bridge(s). The A subunit forms the binding site for TSH on the outside of the cell membrane. The B subunit appears to penetrate the membrane and form the site for interaction with adenylate cyclase either in the lipid bilayer or close to the cytoplasmic surface of the membrane.

Assessment of the shape and molecular size of TSH-TSH receptor complexes

Photoaffinity labelling and analysis under denaturing conditions (SDS-PAGE) have shown that the porcine TSH receptor contains an A subunit (Mr = 47,000) which forms the binding site for TSH and a B subunit (Mr = 25,000) linked to the A subunit by a disulphide bridge. In order to assess the size and shape of the receptor under non-denaturing conditions we have solubilized photoaffinity-labelled porcine TSH receptors using the small micelle-sized detergent sodium deoxycholate and analysed the preparations by sucrose density gradient centrifugation and gel filtration. Under these conditions, the cross-linked TSH-TSH receptor complex showed an S20,w of 6.4 S and a frictional ratio f/f0 of 1.8. These values were consistent with those which might be expected from an elongated protein complex with a molecular weight of about 100,000 (the value obtained by SDS-PAGE). Analysis of another thyroid membrane protein, human thyroid microsomal antigen (Mr = 110,000 by SDS-PAGE) under the same conditions gave an S20,w of 6.0 S and f/f0 = 1.3, suggesting that this protein has a compact structure. The TSH receptor A subunit cross-linked to TSH (Mr = 70,000 by SDS-PAGE) gave an S20,w of 4.6 S and f/f0 = 1.8 and these values could be compared with those obtained for the A subunit alone (S20,w = 3.6 S; f/f0 = 1.4; Mr by SDS-PAGE = 47,000) and TSH alone (S20,w = 2.6 S; f/f0 = 1.6; Mr = 28,000.(ABSTRACT TRUNCATED AT 250 WORDS)

Isoelectric focusing of the human TSH receptor A subunit

The water soluble A subunit of the human TSH receptor has been shown to have an isoelectric point of 5. As both TSH and TSH receptor antibodies have isoelectric points in the region of 8–10, charge-charge interactions must be of major importance in the binding of hormone or antibody to the TSH receptor A subunit.