Immunologic mapping of the amino- and carboxy-termini of the turkey erythrocyte beta-adrenergic receptor: selective proteolysis of both domains

Expression and purification of truncated, non-glycosylated turkey beta-adrenergic receptors for crystallization

In order to purify milligram quantities of turkey beta-adrenergic receptor (betaAR) for structural analysis, we have expressed mutant betaARs using the baculovirus system. The initial betaAR construct was truncated at both N- and C-termini thus removing an N-glycosylation site. Cys 116 was mutated to leucine and a histidine tag was added at the C-terminus resulting in the betaAR construct 20-424/His6. Expression of this construct in Sf9 cells produced 0.5 mg of unpurified receptor per liter of culture which necessitated the use of a fermenter for large-scale production. The yield was improved more than 2-fold to 1.2 mg/l culture by using Tni cells which facilitated the production of receptor on a 4 litre scale in shake cultures. The receptor was purified to homogeneity with 35% recovery giving a yield of 2 mg receptor. A further deletion at the N-terminus (betaAR 34-424/His6) eliminated proteolysis which had been observed with the original construct and also increased expression more than 5-fold to 360 pmol/mg solubilized membrane protein. This expression level is one of the highest reported for a G protein-coupled receptor (GPCR) and has enabled us to purify 10 mg betaAR for large-scale crystallization experiments.

Probing human beta1- and beta2 -adrenoceptors with domain-specific fusion protein antibodies

In order to generate antibodies suitable for immunological studies on beta-adrenoceptors constitutively expressed at low levels in cells or tissues we have produced fusion proteins of the amino- and carboxy-terminus, and the second extracellular loop of the human beta1- or beta2-adrenoceptors with bacterial glutathione-S-transferase in E. coli. Rabbit antibodies raised against these fusion proteins strongly reacted with intact human beta1- or beta2-adrenoceptors in a subtype- and domain-specific manner. Antibodies directed against the second extracellular loop of the beta1-adrenoceptor reacted stronger with non-denatured receptors and decreased the affinity of the 3H-labelled antagonist (-)-4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]benzimidazol-2-one ([3H]CGP 12 177), indicating a specific interaction with the native receptor. In contrast, antibodies directed against carboxy- and amino-terminal receptor domains reacted strongly both with denatured and non-denatured receptors but did not interfere with binding of [3H]CGP 12 177. Affinity purified antibodies were used for detecting the beta1- or the beta2-adrenoceptor subtype heterologously produced in Sf9 cells by enzyme-linked immunosorbent assay, Western blotting, immunoprecipitation, and indirect immunofluorescence microscopy. Moreover, we could demonstrate that avidity, titers, and specificity of these antibodies were high enough for studying beta-adrenoceptors constitutively expressed in human A431 cells, where we observed a patched membrane distribution of the receptors.

Probing human beta 1- and beta 2-adrenoceptors with domain-specific fusion protein antibodies

In order to generate antibodies suitable for immunological studies on beta-adrenoceptors constitutively expressed at low levels in cells or tissues we have produced fusion proteins of the amino- and carboxy-terminus, and the second extracellular loop of the human beta 1- or beta 2-adrenoceptors with bacterial glutathione-S-transferase in E. coli. Rabbit antibodies raised against these fusion proteins strongly reacted with intact human beta 1- or beta 2-adrenoceptors in a subtype- and domain-specific manner. Antibodies directed against the second extracellular loop of the beta 1-adrenoceptor reacted stronger with non-denatured receptors and decreased the affinity of the 3H-labelled antagonist (-)-4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]benzimidazol-2-one ([3H]CGP 12 177), indicating a specific interaction with the native receptor. In contrast, antibodies directed against carboxy- and amino-terminal receptor domains reacted strongly both with denatured and non-denatured receptors but did not interfere with binding of [3H]CGP 12 177. Affinity purified antibodies were used for detecting the beta 1- or the beta 2-adrenoceptor subtype heterologously produced in Sf9 cells by enzyme-linked immunosorbent assay, Western blotting, immunoprecipitation, and indirect immunofluorescence microscopy. Moreover, we could demonstrate that avidity, titers, and specificity of these antibodies were high enough for studying beta-adrenoceptors constitutively expressed in human A431 cells, where we observed a patched membrane distribution of the receptors.

Photoaffinity labeling of the human brain cholecystokinin receptor overexpressed in insect cells. Solubilization, deglycosylation and purification

The human cholecystokinin B (CCKB) receptor was expressed in Sf9 cells by infection with recombinant baculovirus. For immunodetection a c-myc epitope tag (EQKLISEEDL) was fused at the amino-terminus of the CCKB receptor. In a second construct an additional hexa-histidine tag was introduced at the C-terminus of the CCKB receptor to enable employment of metal affinity chromatography. The two receptor constructs were expressed at densities of 6.0 +/- 1.1 pmol/mg protein and 7.2 +/- 1.1 pmol/mg protein, respectively which are 100-200-fold higher compared with the receptor amounts found in natural sources. Saturation of the binding sites with [3H]propionyl-CCK8 revealed Kd values of 4.5 +/- 0.5 nM and 7.8 +/- 0.6 nM for the CCKB receptor without or with histidine tag. In SDS/PAGE and subsequent immunodetection the histidine-tagged CCKB receptor migrated as a 55-kDa band, whereas the CCKB receptor without C-terminal modification revealed apparent molecular masses of 45 kDa and 49 kDa. The differences in the mass values observed for the two constructs suggest that the histidine tag could protect the CCKB receptor against proteolytical degradation from its C-terminus. Furthermore two new photoreactive derivatives of cholecystokinin octapeptide residues 26-33 (CCK8) with high labeling efficiency and specificity for the cholecystokinin receptor subtype B were developed: [3H]BzBz-des-Met28-[p-NH2Bz29]-CCK8 and [3H]BzBz-biotinyl-des-Met28-[p-NH2Bz29]-CCK8. Both contain the p-benzoyl-benzoyl (BzBz) residue at the N-terminus for photoactivation and a p-aminobenzoyl (p-NH2BZ) residue instead of Met28-Gly29 in cholecystokinin. Enzymatic deglycosylation of the CCKB receptor with N-glycosidase F after photoaffinity labeling demonstrated that the CCKB receptor with three potential glycosylation sites was slightly glycosylated, amounting to a molecular mass of about 4 kDa. Using the biotinylated cholecystokinin derivative the photoaffinity-labeled CCKB receptor could be purified 1260-fold by a two-step procedure including affinity chromatography on a streptavidin/avidin agarose matrix. For purification of the native receptor, an improved solubilization protocol for the CCKB receptor using dodecyl beta-D-maltopyranoside was developed. The solubilized CCKB receptors with C-terminal histidine tag retained their ligand binding characteristics after chromatography on a nickel affinity matrix.

Anti-peptide monoclonal antibodies to the beta-adrenergic receptor: use in purification of beta receptor

This article describes a new immunopurification procedure based on monoclonal antibodies raised against peptides of the carboxy-terminal region of the turkey beta-adrenergic receptor. This procedure constitutes a significant purification step of recombinant beta-adrenergic receptors expressed in baculovirus-infected Sf9 cells, and allows the recovery of receptors able to activate Gs in phospholipid vesicles. Additionally, this procedure can be combined with affinity chromatography to yield nearly homogeneous receptor.

The carboxyl-terminal anchorage domain of the turkey beta 1-adrenergic receptor is encoded by an alternatively spliced exon

The originally described cDNA of the turkey beta 1-adrenergic receptor encodes a receptor with a carboxyl-terminal, 59-amino acid extension that was not found in several mammalian beta 1-adrenergic receptors. This extension blocks agonist-promoted endocytosis and down-regulation of the receptor. This carboxyl-terminal domain is encoded by an exon distinct from that which encodes the body of the receptor, and the originally described cDNA results from removal of an 849-nucleotide intron. Unspliced mRNA encodes a shorter open reading frame whose translated carboxyl terminus is identical with that of the mammalian beta 1-adrenergic receptors. There is no evidence for other introns in the coding region. Splicing of the intron to produce the non-endocytosing receptor is highest in fetal blood cells, is appreciable in adult brain and heart, and is detectable in other tissues. Thus, different tissues use alternative splicing to express beta-adrenergic receptors that either do or do not endocytose and down-regulate in response to agonist.

Olfactory receptor proteins. Expression, characterization and partial purification

A rat olfactory epithelium cDNA library was screened for olfactory receptor clones. One of the positively hybridizing cDNA clones was sequenced and found to encode a new member of the olfactory receptor superfamily. This cDNA, termed olp4, was used as a model of olfactory receptor for expression, both in vitro and in vivo. Expression of olp4, as well as of another previously cloned olfactory receptor (F5), was monitored by immunoprecipitation was a monoclonal antibody directed against a Flag peptide epitope tag, inserted at the N-terminus of the open reading frame, and a specific polyclonal antibody against a C-terminal peptide of olp4. Translation in vitro, followed by immunoprecipitation, showed a major olp4-specific band of 27-29 kDa. The olp4 and F5 polypeptides were found to be inserted into microsomal membranes as expected for integral membrane proteins. Expression in vivo of Flag-olp4 in Sf9 insect cells, using the baculovirus expression system, showed a specific polypeptide of the same size as the in vitro species, with an additional band of 34 kDa, which is most likely a glycosylated form. Fluorescence cytometry and immunohistochemical assays demonstrated the localization of the Flag-olp4 product on the cell surface of the infected host Sf9 cells, with the N-terminus and C-terminus in the proper orientation. Affinity chromatography was used for the partial purification of the olp4 polypeptide from infected Sf9 cells. The identification and purification of this expressed olfactory receptor polypeptide could open the way for further characterization and functional studies of the olfactory receptor superfamily members.

D2 dopamine receptor protein location: Golgi impregnation-gold toned and ultrastructural analysis of the rat neostriatum

The neostriatal distribution of D2 dopamine receptor protein has been assessed using subtype-selective polyclonal antibodies generated against three unique polypeptide sequences of the receptor. The experimental tissues were processed by peroxidase based immunohistochemical procedures for routine light microscopy, Golgi impregnation-gold toned morphological characterization, and correlative light/electron microscopy. The results demonstrated a regional gradient of D2-like dopamine receptor expression in the neostriatum, where lateral portions in the nucleus exhibited more reactive cell bodies than medial portions. D2-like expression was detected in the three populations of neostriatal neurons, i.e., the medium-sized spiny projection neurons, and the medium- and large-sized aspiny interneuron types. Morphometric measurements of labeled neurons verified that medium and large diameter neurons expressed the D2-like receptor subtype. D2-like immunoreactivity was distributed throughout the cytoplasm in dendritic processes, and in presynaptic terminal boutons. Immunoreactivity for the receptor protein was also detected in small, thinly myelinated axons, suggesting the possibilities of anterograde transport of the receptor from cell bodies in the substantia nigra to their neostriatal terminal fields, as well as from local axon collaterals of neostriatal projections neurons. These findings provide evidence of widespread distribution of the D2-like receptor protein in neostriatal neurons, and showed that the presynaptic D2 receptors contain analogous epitopes to the postsynaptic receptor subtype.

Dopamine receptor distribution in the rat CNS: elucidation using anti-peptide antisera directed against D1A and D3 subtypes

Anti-peptide antibodies were generated against amino acid sequences of intracellular and extracellular portions of the native proteins for the cloned rat D1A and D3 dopamine receptor subtypes in order to determine the cellular distribution of these specific forms in the brain. These polyclonal antisera exhibited high specific titers, assessed by ELISA and immunofluorescent detection of functional recombinant receptor proteins expressed in stably transfected Chinese hamster ovary (CHO) cells. Central nervous system (CNS) areas of the male rat were examined using standard immunofluorescent methods in fresh frozen tissues. This paradigm detected D1A-like and D3-like dopamine receptor staining primarily in larger-sized neurons throughout layers 3 and 5 of the cortex, in medium-diameter somata of the striatum, and in the densely packed cells of the olfactory tubercle and hippocampal formation. More attenuated immunoreactivity for both dopamine receptor subtypes was noted in the substantia nigra, not associated with perikarya. Differences in cellular staining patterns and intensity were evident between the D1A-like and D3-like dopamine receptor subtypes. Equivalent morphological elements exhibited dopamine receptor expression following incubation using antisera generated against either extracellular or intracellular epitopes of either the D1A or D3 native proteins. Dopamine receptor immunoreactivity could not be detected in the cerebellum at equivalent antisera dilutions used to discriminate cellular staining patterns within the forebrain. Fluorescent-labeled latex microspheres were infused into the substantia nigra terminal fields to retrogradely identify the cell bodies of the striatonigral projection system. This paradigm showed that 80% of striatonigral neurons expressed D1A-like receptors, while 65% demonstrated D3-like dopamine receptor staining. This distribution for the D1A-like and D3-like receptor subtypes suggests that overlap may occur in the expression of the receptors in the striatonigral neuron population. Our previous results localizing cellular D2-like receptor expression patterns in this projection system of the rat neostriatum implies that all three of these dopamine receptor subtypes may be co-expressed in this efferent system.