Use of the heterobifunctional cross-linker m-maleimidobenzoyl N-hydroxysuccinimide ester to affinity label cholecystokinin binding proteins on rat pancreatic plasma membranes

Top-down synthesis of versatile polyaspartamide linkers for single-step protein conjugation to materials

Materials used in various biological applications are often modified with proteins to regulate biomolecular and cellular adhesion. Conventional strategies of protein conjugation accompany monovalent bifunctional protein linkers, which present several limitations in molecular synthesis and protein conjugation. Herein, we present a new strategy of preparing multivalent polyaspartamide linkers in a simple top-down manner, and also demonstrate that the resulting polymer linkers allow us to readily conjugate proteins to both organic and inorganic materials. The top-down synthesis of polyaspartamide linkers was performed by partially opening succinimidyl ring moieties of polysuccinimide (PSI) with the controlled number of nucleophiles reactive to photo-cross-linked hydrogel or gold-coated inorganic materials: (1) Poly(2-hydroxyethyl-co-2-methacryloxyethyl aspartamide) (PHMAA) presenting methacrylate was used to micropattern fibronectin or collagen on a hydrogel in order to regulate cell adhesion and growth area on a micrometer scale. (2) Poly(2-hydroxyethyl-co-2-mercaptoethyl aspartamide) (PHMCA) presenting thiol functional groups was used to link fibronectin to a gold-coated silicon microelectromechanical probe designed to measure cell traction force. Overall, these multivalent polyaspartamide protein linkers will greatly assist efforts to analyze and regulate the cellular adhesion to and phenotypic activities of a wide array of substrates and devices.

Synthesis of the fatty sterol bound protein for a new sterol antibody

For the purpose of applying the particular antibodies as a new diagnostic procedure for atherosclerosis and related diseases, we successfully achieved the synthesis of the fatty sterol with a linker, then linked the target protein to this sterol. Synthesis was started from pregnenolone and achieved by the Grignard reaction with pentenyl magnesium bromide, regioselective photoaddition of thiolacetic acid toward the 25-double bond, esterification of 3-OH with linoleic anhydride, in situ conjunction of the cross-linker (MBS) to the thiol group after selective deprotection from its acetyl ester, and finally by the reaction with protein such as KLH or albumin through this linker.

Characterization of a novel receptor in toad retina with dual specificity for insulin and insulin-like growth factor I

The biochemical properties of insulin receptors from toad retinal membranes were examined in an effort to gain insight into the role this receptor plays in the retina. Competition binding assays revealed that toad retinal membranes contained binding sites that displayed an equal affinity for insulin and insulin-like growth factor I (IGF-I). Affinity labeling of toad retinal membrane proteins with 125I-insulin resulted in the specific labeling of insulin receptor alpha-subunits of approximately 105 kDa. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of partially reduced (alpha beta-heterodimer) receptors affinity-labeled with 125I-insulin indicated the presence of a disulfide-linked beta-subunit of approximately 95 kDa. Endoglycosidase F digestion of the affinity-labeled alpha-subunits increased their mobility by reducing their apparent mass to approximately 83 kDa. This receptor was not detected by immunoblot analysis with a site-specific antipeptide antibody directed against residues 657-670 of the carboxy terminal of the human insulin receptor alpha-subunit, whereas this antibody did label insulin receptor alpha-subunits from pig, cow, rabbit, and chick retinas. In in vitro autophosphorylation assays insulin stimulated the tyrosine phosphorylation of toad retina insulin receptor beta-subunits. These data indicate that toad retinal insulin receptors have a heterotetrameric structure whose alpha-subunits are smaller than other previously reported neuronal insulin receptors. They further suggest that a single receptor may account for both the insulin and IGF-I binding activities associated with toad retinal membranes.

Protease peptide mapping of affinity-labeled rat pancreatic cholecystokinin-binding proteins

Affinity-labeling probes with sites of cross-linking distributed along the ligand have been used to biochemically characterize the pancreatic cholecystokinin (CCK) receptor. Probes with photolabile sites spanning the receptor-binding domain have labeled a Mr = 85,000-95,000 plasma membrane protein, while a probe cross-linked via the amino terminus of CCK-33, far removed from the carboxyl-terminal receptor-binding domain, has labeled a distinct Mr = 80,000 protein. In this work, protease peptide mapping of the pancreatic proteins labeled by each of these probes has been performed to gain insight into the identities of the bands and to define domains of the labeled proteins. Photolabile decapeptide probes with sites of cross-linking at the amino terminus, mid region, and carboxyl terminus of the receptor-binding domain each labeled a Mr = 85,000-95,000 glycoprotein with a Mr = 42,000 core protein and similar Staphylococcus aureus V8 protease peptide maps. This confirms that each probe labels the same binding protein and the same domain of that protein. Serial slices through the broad labeled band were separately deglycosylated and protease-treated, demonstrating a single protein core with differential glycosylation. The CCK-33-based probe, however, labeled predominantly two proteins, one having similar sizes in its native and deglycosylated forms to that labeled by the decapeptide probes and a distinct Mr = 80,000 protein. Of note, the peptide map of the protein believed to be the same as that labeled by the shorter probes was different, suggesting that this probe labeled the binding subunit at a site distinct from that which was labeled by the short probes.

Biochemical properties of brain somatostatin receptors

The physical properties of brain and pituitary somatostatin receptors were characterized using photocrosslinking techniques. Somatostatin receptors in rat corpus striatum and anterior pituitary membranes were covalently bound to the non-reducible somatostatin analog, [125I]CGP 23996, using the crosslinking agent n-hydroxysuccinimidyl-4-azidobenzoate and ultraviolet light. In striatal membranes, a protein of 60,000 mol. wt was labeled by [125I]CGP 23996. The binding was potently inhibited by somatostatin analogs but not by other biologically active peptides. The labeling of the 60,000 mol. wt protein by [125I]CGP 23996 was diminished by guanine triphosphate gamma thiol, which is consistent with the labeling of a somatostatin receptor coupled to guanine triphosphate binding proteins. The migration of the [125I]CGP 23996 labeled 60,000 mol. wt protein in native sodium dodecyl sulfate-gels was not affected by the reducing agent dithiothreitol, indicating that there is a general lack of disulfide bridges in the striatal somatostatin receptor. The striatal somatostatin receptor was solubilized with the detergent 3-[(3-cholamidopropyl)-dimethylaminoio]-1-propanesulfonate and specifically bound to the lectin wheat germ agglutinin, suggesting that the striatal somatostatin receptor is a glycoprotein. [125I]CGP 23996 also labeled a 60,000 mol. wt protein in anterior pituitary membranes. The characteristics of [125I]CGP 23996 binding to anterior pituitary membranes were consistent with the labeling of a somatostatin receptor. Interestingly, a comparison of the [125I]CGP 23996 labeled material from striatal and anterior pituitary membranes by two-dimensional polyacrylamide gel electrophoresis revealed the presence of several striatal somatostatin receptors of varying charge (pI values between 6 and 6.5) but only a single pituitary receptor. These findings indicate that physical differences may exist between subtypes of somatostatin receptors.

Use of N,O-bis-Fmoc-D-Tyr-ONSu for introduction of an oxidative iodination site into cholecystokinin family peptides

We report the synthesis of a new reagent for the introduction of an oxidative iodination site into the amino terminus of acid-labile peptides, and the use of this reagent to synthesize a novel affinity-labeling probe for the cholecystokinin (CCK) receptor. The acylation reagent, N,O-bis-fluorenylmethyloxycarbonyl-D-tyrosine hydroxysuccinimide ester, utilizes base-labile protection of both the alpha amino group and the aromatic ring hydroxyl. This can be safely removed to expose a cross-linkable free amino group on the aminopeptidase-resistant D-enantiomer of tyrosine. The synthetic probe, D-Tyr-Gly-Asp-Tyr(OSO3H)-Nle-Gly-Trp-Nle-Asp-Phe-NH2, was fully biologically active, could be radioiodinated to high-specific radioactivity (2000 Ci/mmol), bound with high affinity to the pancreatic CCK receptor, and covalently labeled the hormone-binding site. This reagent should be useful for the synthesis of a wide variety of analogues of CCK and other acid-labile peptides.

Characterization of cholecystokinin receptors in toad retina

The binding characteristics, structure, and pharmacologic properties of a cholecystokinin binding protein in toad retinal membranes have been studied. In competition binding studies using 125I-CCK-8, toad retinal membranes exhibited a high affinity binding site having a Ki50 of 1.5 nM using CCK-8 as competitive ligand. The relative potencies of CCK-related peptides in inhibiting radioligand binding were caerulein greater than gastrin II approximately equal to CCK-8 approximately equal to CCK-33 greater than CCK-8-DS approximately equal to gastrin I. L-364,718, a potent inhibitor of peripheral CCK receptors, was ineffective at competition binding at concentrations up to 1 microM; dibutyryl cyclic GMP was modestly effective at competing (KD approximately 10 mM). Covalent binding of 125I-CCK-33 to toad retinal membranes using chemical cross-linkers or UV irradiation resulted in the labeling of a major Mr 62,000 protein and the intermittent labeling of minor components of Mr 105,000 and Mr 40,000 as determined by SDS-PAGE and autoradiography. The binding of 125I-CCK-33 to retinal membranes and the concomitant labeling of the Mr 62,000 component was specifically inhibited by CCK-8 (KD approximately 1.5 nM). Reduction of membranes with DTT abolished specific binding of 125I-CCK. SDS-PAGE analysis of affinity cross-linked membranes under non-reducing conditions revealed that the Mr 62,000 protein migrated with an apparently lower molecular weight. These results suggest that the Mr 62,000 CCK binding protein in the toad retina contains an intramolecular disulfide bond(s). The Mr 62,000 protein was retained on a wheat germ agglutinin-agarose column and eluted with N-acetyl D-glucosamine, suggesting the glycoprotein nature of this protein. Digestion of the Mr 62,000 protein with neuraminidase together with O-glycanase resulted in a discrete product of Mr approximately 60,000. These results indicate that the Mr 62,000 protein is a glycoprotein with O-linked oligosaccharide chains. Taken together, these data indicate that the CCK receptor in toad retina has a distinct structure compared to that described in rat pancreas or brain. It will be important to establish whether this difference is reflected in differences in signal transduction mechanisms.

Biochemical characterization of the pancreatic cholecystokinin receptor: a possible marker of cell differentiation and development

The biochemical expression of the cholecystokinin (CCK) receptor on the surface of the pancreatic acinar cell is a potential marker of the state of differentiation of that cell. In this report we review the basis for and the results of the use of affinity labeling techniques for the biochemical characterization of this macromolecular receptor assembly on the adult rat pancreatic acinar cell. A series of specially designed molecular probes are used to define the subunit structure of this receptor, based on the relationships between the sites of covalent attachment of these probes and their receptor-binding domains. We suggest that the receptor-binding domain resides on a Mr = 85,000-95,000 subunit, whereas a distinct Mr = 80,000 also exists as part of this complex.

Establishment of a new short, protease-resistant, affinity labeling reagent for the cholecystokinin receptor

Proteolytic degradation of radioligands is an important source of artifact in affinity labeling of receptor proteins. To complement our previous characterization of the pancreatic acinar cell cholecystokinin (CCK) receptor, we synthesized D-Tyr-Gly[(Nle28,31)CCK-26-33]. The amino terminal D-enantiomer of tyrosine provided a site for oxidative iodination, a free amino group for cross-linking, and rendered the peptide resistant to aminopeptidases. The decapeptide was oxidatively iodinated and purified by reverse-phase HPLC to 2,000 Ci/mmol, to yield a probe which was equal in potency and efficacy to CCK-8, and which bound to rat pancreatic membranes in a rapid, reversible, temperature-dependent, specific, saturable and high affinity manner. This probe was resistant to aminopeptidase degradation, and maintained its ability to bind to receptor after incubation with pancreatic membranes or dispersed cells. Affinity labeling of pancreatic membranes with this analogue identified an Mr = 85,000-95,000 molecule. This analogue offers several advantages over existing probes and should be useful for future studies of this and other CCK receptors.

Receptors for cholecystokinin and gastrin peptides display specific binding properties and are structurally different in guinea-pig and dog pancreas

In the light of the strong potency of gastrin-related peptides on pancreatic exocrine secretion in dog, we analyzed the binding properties of peptides related to cholecystokinin (CCK) and gastrin on dog pancreatic acini compared to guinea-pig acini. Moreover, we determined apparent molecular masses of photoaffinity labelled CCK/gastrin receptors in the two models. Using the CCK radioligand, receptor selectivity towards CCK/gastrin agonists and antagonists was found to be lower in dog acini than in guinea-pig acini. Performing the binding with CCK and gastrin radioligands in combination with N2,O2'-dibutyryl-guanosine 3',5'-monophosphate, revealed that in dog acini there exist two different sub-classes of CCK/gastrin receptors having high and low selectivity, the latter ones being able to bind gastrin with high affinity (Kd = 2.1 nM). SDS-PAGE analysis of covalently cross-linked receptors using several photosensitive CCK and gastrin probes of different peptide chain lengths demonstrated that in guinea-pig, CCK peptides bound to a 84-kDa component whereas in dog pancreas, CCK and gastrin peptides bound to three distinct molecular species (Mr approximately equal to 78,000, 45,000, 28,000). Performing cross-linking in the presence of 1 microM CCK indicated that a 45-kDa protein is the putative CCK/gastrin receptor in dog pancreas. Our results support the concept of heterogeneity of CCK/gastrin receptors.

Characterization of [3H](+/-)L-364,718 binding to solubilized cholecystokinin (CCK) receptors of rat pancreas

The binding of [3H](+/-)L-364,718 (3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine-3-yl )-1H-indole-2-carboxamide), an extremely potent nonpeptide cholecystokinin (CCK) receptor antagonist, to digitonin-solubilized CCK receptors from rat pancreas was characterized. [3H](+/-)L-364,718 binding to digitonin-solubilized receptors was assayed using polyethylene glycol precipitation followed by rapid filtration to separate free and bound [3H](+/-)L-364,718. Specific [3H](+/-)L-364,718 binding to solubilized receptors was dependent on the digitonin and receptor concentration and, under optimal conditions, represented greater than 90% of the total binding. Scatchard analysis indicated a single class of binding sites with a Kd of 0.53 nM and a Bmax of 3.1 pmol/mg protein. Specific [3H](+/-)L-364,718 binding to solubilized CCK receptors was inhibited by both CCK receptor agonists and antagonists in a stereospecific manner. After solubilization, the affinities of various antagonists to displace specific [3H](+/-)L-364,718 binding were similar to those obtained with membrane-bound receptors; however, the affinities of CCK agonists were reduced 10-100 times. Collectively, the data presented indicate that [3H](+/-)L-364,718 represents a new antagonist ligand which has apparent advantages over the agonist ligand [125I]CCK in assaying digitonin-solubilized receptors. Gel filtration of the digitonin-solubilized CCK receptors followed by [3H](+/-)L-364,718 binding determinations revealed an estimated molecular weight of 400,000 daltons.

Heterogeneity of cholecystokinin receptors in pancreas

Specific labeling of a major Mr 85-95 K protein was obtained using the SH, NH2 heterobifunctional cross-linker m-maleimidobenzoyl N-hydroxysuccinimide ester (MBS) to affinity label cholecystokinin (CCK) receptors on rat pancreatic plasma membranes, pancreatic acinar cells and acinar cell tumor membranes with 125I-CCK-33. Endoglycosidase F (endo F) digestion of this species in gel slices indicated that at least two components were present which contain N-linked glycans. The smaller protein of Mr approximately 85 K was digested by endo F to a final product of approximately Mr 62 K while the larger Mr approximately 95 K protein generated two endo F products of Mr 55 K and Mr 43 K. These findings suggest that the receptor for CCK on pancreatic acinar cells exhibits an oligomeric structure, possessing two distinct CCK-binding proteins.

Molecular properties of solubilized CCK receptor from guinea-pig pancreas

In order to characterize the CCK receptor in guinea-pig pancreas, iodinated CCK-39 was bound to pancreatic membranes and the reversible complex was solubilized using various non-denaturing detergents. In term of recovery of ligand stabilized receptors, the relative potencies were Zwittergent 3-14 greater than CHAPS = CHAPSO greater than digitonin greater than MEGA 10 greater than octyl beta-D-glucopyranoside. The stability of receptor complexes was increased by glycerol. Chromatographic analysis revealed that digitonin was the most efficient detergent for disaggregation of CCK receptor complex since it yielded a 76 kDa component in addition to the large components obtained after solubilization with CHAPS and Zwittergent. Furthermore, CCK receptors were covalently labelled using dissuccinimidyl suberate or UV irradiation of labelled membranes by photoactivable radioiodinated CCK-39 and subsequently solubilized by CHAPS + SDS or by SDS alone. A predominant molecule was characterized by chromatography (76 kDa) and SDS-PAGE (89 kDa). In addition to this component, other components having molecular masses of 130-150 kDa, 57 kDa and 40 kDa were detected by SDS-PAGE. They correspond to minor bands. These bands, except the 40 kDa band, were protected from covalent labelling by the presence of CCK-39 (10(-6) M) during initial incubation. Reduction under beta-mercaptoethanol mainly resulted in the decrease of high molecular weight aggregates (Mr greater than 200 kDa). We concluded that for a given detergent a specific molecular weight pattern of solubilized CCK receptor complex is achieved. The minimal component had a molecular mass of 71-84 kDa according to the method of biochemical analysis used.