Structure of the thyrotropin receptor and thyroid adenylate cyclase system as determined by target analysis

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Abstract
Bovine thyroid plasma membranes were irradiated with high-energy electrons. Analysis of the target size of the thyrotropin (TSH) receptor revealed a complex pattern composed of a TSH binding component of 71 kD [kilodalton] and a large component (several hundred thousand daltons) that masked some of the binding. Both components were also observed when binding was assayed in the presence of 50 mM NaCl. Membranes preincubated with Mg2+ and 10 .mu.M guanosine 5''-(.beta.,.gamma.-imidotriphosphate) [Gpp(NH)p], a persistent activator of adenylate cyclase, also showed the presence of these same components. Although the receptor for TSH has been reported to have some similarities to the receptor for cholera toxin, target analysis of [125I]iodocholera toxin binding was consistent with a single small component about the size of a ganglioside. Measurement of the target size of ground state, i.e., not preactivated, adenylate cyclase was also carried out. The basal (Mn2+) adenylate cyclase yielded a MW of 85,000, the smallest unit capable of producing cAMP. The Gpp(NH)p-responsive adenylate cyclase has as MW of 150,000, which may reflect the contribution of the guanine nucleotide regulatory component to the mass of the active enzyme. A similar size was previously measured for the Gpp(NH)p-preactivated, detergent-solubilized thyroid enzyme. Radiation inactivation of the NaF-responsive enzyme indicated .gtoreq. 2 components to this activity, the smaller of which (140 kD was similar in size to the ground-state Gpp(NH)p-responsive enzyme and the larger of which was > 106 daltons. TSH stimulation of adenylate cyclase was also consistent with the presence of .gtoreq. components, the smallest being 240 K and the largest being > 106 daltons. These structures may correspond to a contribution of the hormone receptor (71 kD) and the combination of nucleotide regulatory component and catalytic unit (150 kD). Thus, the structure of the thyroid adenylate cyclase appears to be as complex as the adenylate cyclase from rat liver and more complex than turkey erythrocyte adenylate cyclase.