Photoactivatable, and Cleavable Through an Azo Linkage
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Proc. Nad. Acad. Sci. USA Vol. 81, pp. 5286-5290, September 1984 Biochemistry "25I-labeled crosslinking reagent that is hydrophilic, photoactivatable, and cleavable through an azo linkage (heterobifunctional reagent/carrier-free specific activity/sulfosuccinimide ester/aryl azide/transfer of radioactive label from derivatized protein A to serum IgG) JOHN B. DENNY AND GUNTER BLOBEL Laboratory of Cell Biology, The Rockefeller University, New York, NY 10021 Contributed by Gunter Blobel, March 7, 1984 ABSTRACT A radioactive crosslinking reagent, N-[4-(P- protein-SH groups (13, 14). In addition, they cannot be used azido-m-[1251]iodophenylazo)benzoyl]-3-aminopropyl- in the presence of thiol reducing agents. We therefore have N'-oxysulfosuccinimide ester, has been synthesized. The rea- prepared a photoactivatable crosslinker that is cleavable gent is photoactivatable, water-soluble, cleavable through an through an azo linkage, as in reagents described previously azo linkage, and labeled with 125I at the carrier-free specific (15-17), but with the added advantages of the high specific activity of 2000 Ci/mtnol. Any protein derivatized with the radioactivity of 125I and a sulfonate group to render the com- reagent is thus converted into an '251-labeled photoaffinity pound water-soluble. Photoactivatable crosslinking reagents probe. Crosslinks are formed following photolysis with 366- have been prepared that contain 35S (18) and 125I (19), but in nm light, and cleavage by sodium dithionite results in the do- the latter case the reagents were not cleavable and were not nation of radioactivity to the distal partner in crosslinked com- prepared at the carrier-free specific activity of 2000 Ci/mmol plexes. The newly labeled proteins are then analyzed by gel (1 Ci = 37 GBq). electrophoresis and autoradiography. The compound was pre- In this paper we have selected the protein A-IgG interac- pared by iodination of N-[4-(p-aminophenylazo)benzoylJ-3- tion (20) as a model system with which to test the crosslink- aminopropionic acid using carrier-free Na'25I and chlora- ing reagent. We show that radioactive label is transferred to mine-T, followed by azide formation and conversion to the wa- the heavy chain of IgG when derivatized protein A-Sepha- ter-soluble sulfosuccinimide ester. As a model system, protein rose is photolyzed in the presence of human serum and sub- A-Sepharose was derivatized with the reagent under subdued sequently treated with sodium dithionite. In addition, the ra- light. Each derivatized protein A molecule contained only one dioactive labeling that occurs following photolysis with 366- crosslinker. The derivatized protein A-Sepharose was then nm light is due to reactions mediated by the aryl nitrene and photolyzed in the presence of human serum and subsequently is not due to the incorporation of free 125I. This latter result treated with sodium dithionite. Analysis of the serum by gel is in agreement with those of others (19, 21-24). electrophoresis revealed that 1.1% of the radioactive label originally present on the protein A-Sepharose was transferred MATERIALS AND METHODS to the heavy chain of IgG, which was the most intensely labeled Aniline was purchased from Aldrich and was vacuum-dis- protein in the gel. The next most intensely labeled protein was tilled at 78°C prior to use. Tetrahydrofuran (gold label), diox- IgG light chain, which incorporated radioactivity that was ane (gold label), formaldehyde sodium bisulfite addition lowver by a factor of 3.6 than that of the heavy chain. These compound, p-aminobenzoic acid, sodium nitrite, and sodium results demonstrated the specificity of the derivatized protein azide were purchased from Aldrich and used without further A-Sepharose as a photoaffinity probe. Photolabeling of IgG purification. Dicyclohexylcarbodiimide and N-hydroxysuc- was the result ofnitrene-mediated reactions and was not due to cinimide were from Pierce, Na 251 was from Amersham, so- the incorporation of free 1251. dium dithionite and chloramine-T were from Fisher, TLC plates were from EM Laboratories (Elmsford, NY), and pro- Photoactivatable crosslinking reagents have been success- tein A-Sepharose CL-4B and f-alanine were from Sigma. fully used to detect plasma membrane receptors for insulin NMR spectra were obtained on a Nicolet Fourier trans- (1), epidermal growth factor (2), human choriogonadotropin form spectrometer at 300 MHz. Chemical shifts were rela- (3), and the N-formyl chemotactic peptide (4) and to identify tive to an internal tetramethylsilane standard. IR spectra the nearest neighbors of many proteins, including fibronec- were obtained on a Perkin-Elmer model 237 B spectropho- tin (5), calmodulin (6), fibrinogen (7), glycopeptides (8), and tometer. ribosomal proteins (9) (for other previous studies, see refs. Preparation of N-[4-(p-Aminophenylazo)benzdyl]-3-gmino- 10-12). In some cases (1-4, 6, 8) relatively small proteins propionic Acid (VII). The reaction scheme for the prepara- and peptides were labeled with 125I, derivatized with a pho- tion of compound VII is shown in Fig. 1. toactivatable reagent, and crosslinked to their putative re- Sodium anilino N-methylenesulfohate (III) was prepared ceptors. However, crosslinks were not cleaved and high mo- by mixing 22 g (0.231 mol) of aniline (I) with 33.3 g (0.237 lecular weight complexes were assumed to contain the 125I- mol) of formaldehyde sodium bisulfite addition compound labeled peptide and its receptor. The above approach is more (II) in 100 ml of water at 70°C for 20 min with constant stir- difficult when used to detect receptors for larger pro- ring. The solution was then cooled on ice and the white crys- teins, due to the inability of crosslinked complexes to enter tals were collected by filtration, washed with ether, and vac- polyacrylamide gels. Cleavable, photoactivatable crosslink- uum-dried (yield, 66%). The N-methylenesulfonate group ing reagents were therefore developed (10-12), but most of protected the amino group from unwanted side reactions the reagents are cleavable through a disulfide bond and are during subsequent steps. therefore subject to mercaptan-disulfide interchange with A solution of diazotized p-aminobenzoic acid (IV) was prepared by dropwise addition of 3.6 g of sodium nitrite in 10 The publication costs of this article were defrayed in part by page charge ml of cold (4°C) water to an ice-cold suspension of 6.5 g payment. This article must therefore be hereby marked "advertisement" (0.047 mol) of p-aminobenzoic acid in 27.5 ml of 5.5 M HCl. in accordance with 18 U.S.C. §1734 solely to indicate this fact. After 15 min at 0°C, all of the above solution was added with 5286 Downloaded by guest on September 28, 2021 Biochemistry: Denny and Blobel Proc. Natl. Acad ScL USA 81 (1984) 5287 aminophenylazo)benzoyl]-3-aminopropionic acid (VII), was Ki6@> INH2 + HOCH2SO3 Na+ collected by centrifugation, washed with water, and vacu- (II) um-dried. The yield was 0.163 g (0.52 mmol) or 13% based (I) on the amount of compound VI used. The product at this stage was contaminated with 1-2% of @" NHcH2SO3 Na+ (III) compound VI. TLC on silica gel 60 in chloroform/methanol, 2:1 (vol/vol), was used to purify the desired compound VII (Rf = 0.45) from the contaminant VI (Rf = 0.66). NMR (com- HOOC A g2 i pound VII) (dimethyl-d6 sulfoxide): 2.52 (t, 2H, CH2), 3.49 (IV) /A (q, 2H, CH2), 6.25 (s, 2H, aromatic NH2), 6.71 (d, 2H, aro- HOOC N-N j NHCH2SO3 Na+ (V) matic H), 7.71 (d, 2H, aromatic H), 7.81 (d, 2H, aromatic H), 7.99 (d, 2H, aromatic H), 8.65 (s, 1H, amide-NH) ppm (s = 1 1. NaOH 2. HC1 singlet, d = doublet, t = triplet, q = quartet). IR (compound VII) (potassium bromide): 3460 cm-' (N-H stretch of CONH), 3405 and 3375 cm-' (N-H HOOC ( >)N-N < NH2 (VI) stretch of NH2), 1760 cm-' (C==O stretch of COOH), 1605 cm'1 (N-H bend of NH2). | 1. DCC/POSu Preparation of N-[4-(p-Azido-m-[125I~iodophenylazo)ben- 2. P-alanine (pH 9.5) 3. HC1 zoyl]-3-aminopropyl-N'-oxysulfosuccinimide Ester (X). The reaction scheme for the preparation of the final crosslinker, compound X, is shown in Fig. 2. HOOCCH2CH2HN8 N-N NH2 (VII) To iodinate compound VII, 150 ,ul of a 5 mM solution of the compound in dioxane/water, 2:1 (vol/vol), was mixed FIG. 1. Preparation of N-[4-(p-aminophenylazo)benzoyl]-3- with 10 ,ul (1 mCi, 0.5 nmol) of carrier-free Na1251, 89 ,ul of aminopropionic acid (VII). Aniline (I) reacts with formaldehyde so- water, 30 Al of 0.3 M H2SO4, and 31 ,ul of chloramine-T at 40 dium bisulfite addition compound (II) to give the adduct III. Reac- mg/ml in water (final pH = 1.9). The solution was centri- tion of III with diazotized p-aminobenzoic acid (IV) gives the azo fuged vigorously in a vortex for 5 sec and was then incubated compound (V), which is treated with NaOH to remove the N-methyl for 30 sec at room temperature, followed by the addition of sulfonate group. The resulting azo compound (VI) is converted to an 31 ,ul of NaHSO3 at 30 mg/ml in water. The solution was active ester by using N-hydroxysuccinimide (HOSu) and dicyclo- then immediately extracted three times with 0.5-ml portions hexylcarbodiimide (DCC) and then reacts with 18-alanine to give of ether. The ether extracts were combined and concentrat- compound VII. ed to 50 ,ul with a stream of N2, and the concentrate was then applied to a 0.2-mm-thick silica gel 60 plate with plastic stirring to an ice-cold solution of compound III (13.1 g, 0.063 backing (20 x 20 cm).