Immunoaffinity purification and reconstitution of human alpha(2)-adrenergic receptor subtype C2 into phospholipid vesicles

Large quantities of correctly folded, pure alpha(2)-adrenergic receptor protein are needed for structural analysis. We report here the first efficient method to purify human alpha(2)-adrenergic receptor subtype C2 to homogeneity from recombinant yeast Saccharomyces cerevisiae by one-step purification using a monoclonal antibody column (specific for alpha(2)C2). We show that the adrenoceptor antagonist phentolamine stabilized the receptor during purification. We used a very effective chaotropic agent, NaSCN, to elute the receptor from the immunoaffinity column with an overall yield of 34% before reconstitution. Ligand binding of detergent-solubilized, immunoaffinity-purified receptors could not be demonstrated, but partial recovery of ligand binding activity was achieved when purified receptors were reconstituted into phospholipid vesicles. The reconstituted receptors still bound radioligand after storage on ice for 4 weeks. This purification procedure can be easily scaled-up and thus demonstrates the utility of a monoclonal antibody column and NaSCN elution to purify large quantities of G-protein-coupled receptors.

Receptor and membrane interaction sites on Gbeta. A receptor-derived peptide binds to the carboxyl terminus

The functional organization of Gbetagamma is poorly understood. Regions of bovine brain Gbetagamma that interact with a photoaffinity derivative of an alpha2-adrenergic receptor-derived peptide from the third intracellular loop (diazopyruvoyl-modified peptide Q (DAP-Q)) and a hydrophobic membrane probe (3-trifluoromethyl-3-(m-iodophenyl)diazirine (TID)) were examined. We previously showed that DAP-Q cross-links to specific, competable sites on both the alpha and beta subunits of Go/Gi but not on the gamma subunit and that betagamma subunit was required for stimulation of Go/Gi GTPase activity (Taylor, J. M., Jacob Mosier, G. G., Lawton, R. G., Remmers, A. E., and Neubig, R. R. (1994) J. Biol. Chem. 269, 27618-27624). Similarly, we show here that the membrane-associated photoprobe [125I]TID labels alpha and beta but not gamma. We have now mapped the sites of incorporation of DAP-Q and TID into the beta subunit. TID labels both the 14-kDa amino-terminal and the 23-kDa carboxyl-terminal fragments from a partial tryptic digest of beta while DAP-Q labels only the carboxyl-terminal fragment. Further mapping with endopeptidase Lys C reveals substantial labeling of multiple fragments by TID while DAP-Q labels predominantly a approximately 6-kDa fragment within the carboxyl-terminal 60 amino acids of beta1. Thus, regions within the 7th (or possibly 6th) WD-40 repeat of the beta subunit of G protein interact with the receptor-derived peptide while membrane interaction involves multiple sites throughout the beta subunit.

Introduction of purified alpha 2A-adrenergic receptors into uniformly oriented, unilamellar, phospholipid vesicles: productive coupling to G proteins but lack of receptor-dependent ion transport

Introduction of highly purified alpha 2A-adrenergic receptors (alpha 2AAR) into lipid vesicles resulted in vesicle preparations that were unilamellar in structure, nonleaky to monovalent cations, and uniformly oriented such that the cytoplasmic domains of the alpha 2AAR faced the vesicle exterior. In this orientation, addition of Gi/G(o) G proteins yielded a 4-5-fold stimulation of agonist-dependent guanosine-5'-O-(3-[35S]thio)triphosphate binding to the G protein alpha subunit. These nonleaky, uniformly oriented, alpha 2AAR-containing vesicle preparations allowed us to explore the hypothesis that the alpha 2AAR itself, or in combination with Gi/G(o) proteins, is able to effect ion translocation. Measurements of 22Na+ uptake, 22Na+ efflux, and H+ movement revealed no detectable agonist-stimulated, receptor-dependent, ion translocation, even in the presence of G proteins, suggesting that allosteric regulation of alpha 2AAR by cations and amiloride analogs is not an indication that the alpha 2AAR itself is an ion transporter. Nonetheless, the methodology developed in the present studies for preparation of nonleaky vesicles containing receptor and G proteins should be well suited for evaluating the stoichiometry and selectivity of receptor-G protein interactions and, in particular, G protein specificity in mediating receptor-dependent regulation of voltage-gated or receptor-operated ion channels.

Differential desensitization and phosphorylation of three cloned and transfected alpha 2-adrenergic receptor subtypes

Genes encoding 3 distinct subtypes of human alpha 2-adrenergic receptor are known and are found, respectively, on chromosome 10, 4, and 2 (alpha 2-C10, alpha 2-C4, and alpha 2-C2 adrenergic receptors). All 3 receptors inhibit adenylyl cyclase via Gi proteins. To study and compare their regulatory properties we assessed the ability of each to undergo agonist-promoted desensitization and phosphorylation. When Chinese hamster ovary cells stably expressing each of the three receptor genes were incubated with epinephrine for 20 min, a marked decrease in sensitivity to subsequent agonist-mediated inhibition of adenylyl cyclase was observed for the alpha 2-C10 and alpha 2-C2 receptors but not for the alpha 2-C4 receptors. When similar incubations were performed with 32Pi-labeled cells and the receptors were immunoprecipitated with specific antibodies, alpha 2-C10 and alpha 2-C2 receptors were found to undergo an approximately 3-fold increase in receptor phosphorylation after epinephrine exposure. When transfected into COS cells epinephrine also stimulated phosphorylation of alpha 2-C10 and alpha 2-C2 receptors while having only a slight effect on alpha 2-C4 receptors. Cotransfection of the cells with the cDNA encoding the beta-adrenergic receptor kinase further increased receptor phosphorylation for alpha 2-C10 and alpha 2-C2 receptors while having little or no effect on alpha 2-C4 receptors. Moreover purified and reconstituted recombinant alpha 2-C10 receptors could be phosphorylated in an agonist-dependent fashion whereas alpha 2-C4 receptors could not. These observations suggest receptor subtype-specific differences in susceptibility to regulatory phosphorylation and desensitization.

Regulation of rat renal alpha 2 B-adrenergic receptors by potassium depletion

Potassium depletion and alpha 2-adrenergic receptor (alpha 2-AR) agonists produce similar physiological effects on renal function. Both stimuli increase Na-H exchange in proximal tubule cells, inhibit water transport in collecting tubule cells, and alter blood pressure regulation. The purpose of this study was to determine whether potassium depletion and renal alpha 2-AR subtype expression were linked. Kidney membrane proteins and RNA were harvested from anesthetized rats fed a potassium-deficient diet for 4-20 days (LK 4 to LK 20). Using a selective alpha 2-AR antagonist, [3H]MK-912, we observed that potassium depletion led to a dramatic increase in maximum binding (270% of control) without a change in dissociation constant. Competitive binding studies in LK 14 kidney membranes employing chlorpromazine, prazosin, and oxymetazoline suggested that the increase in alpha 2-ARs in response to potassium depletion was due primarily to an increase in the B subtype of alpha 2-AR. Northern blot analysis demonstrated that renal alpha 2B-AR mRNA levels increased (190% of control) after 4 or 14 days on a potassium-deficient diet. In contrast, there was no difference in steady-state alpha 2A-receptor protein levels by Western blot analysis. We conclude that potassium depletion selectively increases the expression of the B subtype of alpha 2-AR with no detectable effect on alpha 2A-AR expression.

Immunohistochemical localization of alpha 2A-adrenergic receptors in catecholaminergic and other brainstem neurons in the rat

alpha 2-Adrenergic receptors mediate a large portion of the known inhibitory effects of catecholamines on central and peripheral neurons. Molecular cloning studies have established the identity of three alpha 2-adrenergic receptor genes from several species that encode the A, B and C subtypes of the receptor. The rat alpha 2A-adrenergic receptor, as defined by sequence similarity, is the orthologue of the human alpha 2A-adrenergic receptor. In this paper, we report the development of rabbit antisera directed against a portion of the third intracellular loop of the rat alpha 2A-adrenergic receptor and the histochemical localization of alpha 2A-adrenergic receptor-like immunoreactive material in the brainstem and spinal cord of the adult rat. Our antisera detected alpha 2A-adrenergic receptor-specific punctate staining associated with neuronal perikarya. alpha 2A-adrenergic receptor-like immunoreactivity was widely, but heterogeneously, distributed in the brainstem and spinal cord, predominantly in areas involved in the control of autonomic function. Double labelling with antisera to tyrosine hydroxylase or phenylethanolamine-N-methyl-transferase revealed that alpha 2A-adrenergic receptor-like immunoreactivity is present in most, perhaps all, noradrenergic and adrenergic cells of the brainstem. alpha 2A-Adrenergic receptor-like immunoreactivity was detected in a small percentage of the dopaminergic cells of the A9 and A10 groups. This study provides the first description of the specific immunohistochemical localization of alpha 2A-adrenergic receptors using a subtype-specific polyclonal antibody. The results support the view that alpha 2-adrenergic receptors are involved in central cardiovascular control and suggest that the catecholaminergic autoreceptors of central noradrenergic and adrenergic neurons are the A subtype of the alpha 2-adrenergic receptors.