A structural comparison of guinea pig thyroid and fat TSH receptors by photoaffinity labelling

TSH receptors from guinea pig thyroid and epididymal fat have been covalently crosslinked to 125I-labelled TSH conjugated to N-hydroxysuccinimidyl-4-azidobenzoate. Analysis by SDS-PAGE and autoradiography showed bands corresponding to TSH subunits (Mr 14 000) and intact TSH (Mr 28 000; subunits crosslinked) and two at higher Mr separated by 14 000. The latter bands represented one or two subunits of TSH crosslinked to a subunit of the TSH receptor with Mr 57 000 (fat) or 60 000 (thyroid). These Mr values were reduced by trypsin treatment to 43 000 and 50 000, respectively. Analysis under nonreducing conditions showed that both fat and thyroid receptors have a second disulphide linked subunit of Mr 30 000.