Uncoupling of Rat Cerebral Cortical α2‐Adrenoceptors from GTP‐Binding Proteins by N‐Ethylmaleimide

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Abstract
Pretreatment of membranes from rat cerebral cortex with N-ethylmaleimide (NEM) decreased [3H]clonidine binding in a concentration-dependent manner. The Bmax values of high-affinity sites for [3H]clonidine were reduced by 50 μM NEM treatment. Treatment with 500 μM NEM diminished the sum of Bmax of both high-and low-affinity components. GTP, Na+, and Mn2+ exerted little effect on [3H]clonidine binding in NEM-treated membranes. The addition of purified GTP-binding proteins caused an increase in the binding to the membranes pretreated with 50 μM NEM, but did not increase [3H]clonidine binding in membranes treated with 500 μM NEM. In contrast, NEM pretreatment inhibited islet activating protein (IAP)-catalyzed ADP ribosylation of membrane-bound (41,000-dalton) and purified (39,000/41,000-dalton) GTP-binding proteins. From these results, it is suggested that two or three categories of essential sulfhydryl groups are involved in the coupling between agonist, α2-adrenoceptor, and GTP-binding protein. One is a highly sensitive site to NEM (a concentration range of 1–50 μM), which is probably a cysteine residue, IAP-catalyzed ADP-ribosylating site on the α-subunit of GTP-binding protein. Other sites have low sensitivity to NEM (a concentration range of 0.1–1 mM), and are the binding domain of agonist and/or the coupling domain of GTP-binding protein on the α2-adrenoceptor. In addition, Ki-ras p21 protein may lack the capacity to couple with the α2-adrenoceptor.