Receptor affinity chromatography

Biotinylation of proteins in solution and on cell surfaces

This unit contains three protocols for biotinylation of isolated proteins: attaching biotin to primary amines (e.g., amino groups of lysyl residues); attaching biotin to sulfhydryls (i.e., thiol groups of cysteinyl residues); and attaching biotin to carbohydrate residues on proteins. As biotinylation of lysyl and cysteinyl residues may alter protein function, modification of protein carbohydrates, which is usually innocuous, may be preferable if intact protein function is required (e.g., for activity assays or affinity purification). Biotinylation of cysteinyl thiols requires that disulfide bonds in isolated proteins be reduced before labeling. Biotinylation of surface proteins on living cells is also described using mild reaction conditions. Finally, this unit includes a brief description of methods for detecting biotinylated proteins.

Coupling of antibodies with biotin

The avidin-biotin bond is the strongest known biological interaction between a ligand and a protein (Kd = 1.3 x 10-15 M at pH 5.0) (1). The affinity is so high that the avidin-biotin complex is extremely resistant to any type of denaturing agent (2). Biotin (see Fig. 1) is a small, hydrophobic molecule that functions as a coenzyme of carboxylases (3). It is present in all living cells. Avidin is a tetrameric glycoprotein of 66,000-68,000 molecular weight, found in egg albumin and in avian tissues. The interaction between avidin and biotin occurs rapidly, and the stability of the complex has prompted its use for in situ attachment of labels in a broad variety of applications, including immunoassays, DNA hybridization (4-6), and localization of antigens in cells and tissues (7). Avidin has an isoelectric point of 10.5. Because of its positively charged residues and its oligosaccharide component, consisting mostly of mannose and glucosamine (8), avidin can interact nonspecifically with negative charges on cell surfaces and nucleic acids, or with membrane sugar receptors. At times, this causes background problems in histochemical and cytochemical applications. Streptavidin, a near-neutral, biotin-binding protein (9) isolated from the culture medium of Streptomyces avidinii, is a tetrameric nonglycosylated analog of avidin with a molecular weight of about 60,000. Like avidin, each molecule of streptavidin binds four molecules of biotin, with a similar dissociation constant. The two proteins have about 33% sequence homology, and tryptophan residues seem to be involved in their biotin-binding sites (10,11). In general, streptavidin gives less background problems than avidin. This protein, however, contains a tripeptide sequence Arg-Tyr-Asp (RYD) that apparently mimics the binding sequence of fibronectin Arg-Gly-Asp (RGD), a universal recognition domain of the extracellular matrix that specifically promotes cell adhesion. Consequently, the streptavidin-cell-surface interaction causes high background in certain applications (12).

The preparation of biotinyl-epsilon-aminocaproylated forms of the vasoactive intestinal polypeptide (VIP) as probes for the VIP receptor

Vasoactive intestinal polypeptide (VIP) was biotinyl-epsilon-aminocaproylated using sulfosuccinimidyl-6-(biotinamido) hexanoate thereby producing a series of products that were separated by high performance liquid chromatography (HPLC). Seven VIP-derivatives were isolated and the number and location of biotinyl-epsilon-aminocaproylation was determined by a combination of enzymatic degradation and plasma desorption mass spectrometry (PDMS). Receptor binding experiments with the VIP biotinyl-epsilon-aminocaproylated derivatives revealed IC50 values for the monobiotinyl-epsilon-aminocaproylated peptides that were 1.3-3.2 times higher than for natural VIP. All isolated biotinyl-epsilon-aminocaproylated derivatives possess VIP-like bioactivity as shown by an assay measuring pancreatic juice secretion in cat, VIP biotinyl-epsilon-aminocaproylated in position lysine being almost equipotent with natural VIP.

Preparation and characterization of biotinylated probes for the beta-interferon receptor

1. Recently we described the isolation of the beta-interferon receptor [Zhang et al. (1986) J. biol. Chem. 261, 8017-8021]. A highly purified product was obtained but in low quantities. 2. The use of biotinylated beta-interferon as a ligand represents an alternate approach to receptor isolation. 3. We have prepared and characterized the derivatives N-(biotinyl)- and N-(biotinyl-epsilon-aminocaproyl)-recombinant human [Ser17]-interferon beta (B- and BC-recHuIFN beta). 4. Biotin incorporation does not result in any loss of antiviral activity, demonstrating the recognition of the derivative by the cell receptor. 5. The biotinylated recHuIFN beta binds specifically and reversibly to succinoylavidin or guanidine thiocyanate-stripped succinoylavidin linked to a Sepharose matrix. 6. Comparison of the competition curves obtained with [14C]biotin and [3H]biotinyl recHuIFN, in the presence of increasing concentrations of biotin suggests that the IFN moiety of the derivative has little effect on the affinity of biotin for avidin. 7. Biotinylated recHuIFN beta derivatives represent useful probes for the beta-IFN receptor.

Posttranslational processing of the insulin proreceptor

Following translation of the insulin proreceptor by 3T3-L1 adipocytes, about 1.5 hours is required for its conversion into active receptor; an additional 1.5 hours is needed for the active receptor to reach the plasma membrane. During this 3-hour period the proreceptor undergoes a complex series of processing events, glycosylation being an essential processing step. Thus, treatment of 3T3-L1 adipocytes with tunicamycin causes the depletion of cellular insulin binding activity and the accumulation of an inactive aglyco proreceptor. To investigate posttranslational processing of normal proreceptor and the role of glycosylation in active receptor formation, metabolic labeling experiments were conducted. The first 35S-labeled intermediate detected is a 190-kDa polypeptide (proreceptor) which is rapidly (t1/2 = 15 minutes) processed into a 210-kDa species. Both polypeptides contain N-linked core oligosaccharide chains, but in the latter case these chains appear to contain terminal N-acetylglucosamine. The 210-kDa precursor is converted slowly (t1/2 = 2 hours) by proteolytic processing into a 125-kDa (alpha') and 83-kDa (beta') species. Immediately prior to insertion into the plasma membrane, 3 hours after its synthesis, the alpha' and beta' precursors are converted to mature receptor composed of alpha (135 kDa) and beta (95 kDa) subunits. The 125-kDa alpha' and 83-kDa beta' precursors are endoglycosidase H-sensitive and their oligosaccharide chains do not contain terminal sialic acid. Just prior to insertion into the plasma membrane the alpha' and beta' precursors are sialylated, giving rise to the 135-kDa alpha and 95-kDa beta receptor subunits and becoming Endo H resistant and neuraminidase sensitive. In the presence of tunicamycin, a 180-kDa aglyco receptor polypeptide accumulates which is not further processed and does not reach the cell surface. It is concluded that N-linked oligosaccharide chains on the proreceptor are required either for its intracellular translocation to the proteolytic cleavage site or for its identification as a target of the cleavage enzyme. Thus, glycosylation of the insulin proreceptor is crucial for proper processing and formation of functional receptor.