Thomas J. Lobl

On-resin biotinylation of chemically synthesized proteins for one-step purification☆

A general, convenient, one-step purification procedure for chemically synthesized proteins present in low yields using on-resin biotinylation is reported. The protein, terminally deprotected and neutralized on-resin, is stirred in dimethylformamide and then biotinylated with N-hydroxysuccinimidobiotin (2 mg/mg protein on-resin) for 24 h at 45 degrees C. Following low/high hydrogen fluoride cleavage (J. P. Tam, W. F. Heath, and R. B. Merrifield (1983) J. Amer. Chem. Soc. 105, 6442-6455) the crude cleavage product was applied to an avidin agarose column. The column was washed with phosphate-buffered saline until all unbound materials had been eluted off. Then the biotinylated protein was eluted with 0.1 M glycine HCl, pH 2.0. A pilot experiment with two unrelated peptides on-resin established the experimental conditions for biotinylation. We then demonstrated that the chemically synthesized 153 residue [Asp205]-interleukin-1 beta (117-269), present in less than 1% yield in the crude HF cleavage mixture, could be purified to homogeneity in one step. In addition 70 and 114 residue synthetic fragments, (200-269) and (156-269), were also purified in this manner. Biotinylation on-resin appears to be an attractive method of purifying low yield chemically synthesized proteins and for preparing proteins with biotinyl moieties at specific locations such as the amino terminus.

Mapping of MCP‐1 functional domains by peptide analysis and site‐directed mutagenesis

Monocyte chemoattractant protein‐1 (MCP‐1) is a member of the β chemokine family which acts through specific seven transmembrane receptors to recruit monocytes, basophils, and T lymphocytes to sites of inflammation. To identify regions of the human MCP‐1 protein which are important for its biological activity, we have synthesized domain‐specific peptides and tested their ability to antagonize MCP‐1 binding and chemotaxis in THP‐1 cells. We have found that an intercysteine first loop peptide encompassing amino acids 13–35 inhibits MCP‐1 binding and chemotactic activity, while peptides representing the amino‐terminus (amino acids 1–10), second loop (amino acids 37–51), and carboxy‐terminus (amino acids 56–71) of MCP‐1 have no effect. In addition, we have found that cyclization of the first loop peptide by disulfide linkage and blocking the C‐terminus of the peptide by amidation increases the activity of this peptide to block MCP‐1 binding and chemotaxis. In order to specifically identify amino acid residues within the first loop that are crucial for MCP‐1 functional activity, we have substituted alanine for tyrosine (Y13A) or arginine (R18A) in MCP‐1 recombinant proteins. While baculovirus produced wild type and R18A MCP‐1 proteins are indistinguishable in their ability to induce THP‐1 chemotaxis and show modest effects in binding activity compared to commercially available recombinant MCP‐1 protein, the Y13A point mutation causes a dramatic loss in function. The identification of functional domains of MCP‐1 will assist in the design of MCP‐1 receptor antagonists which may be clinically beneficial in a number of inflammatory diseases.

Characterization of steroid binding specificity of the androgen receptor in human foreskin fibroblasts.

Our objective was to evaluate a convenient in vitro model for measuring steroid affinities to the human androgen receptor. The ability of unlabeled analogues of dihydrotestosterone (DHT) to compete with [3H]DHT for binding to the receptor in human fibroblasts was measured and expressed relative to DHT. The C-3 ketone group and the planar configuration of the A and B rings were critical for binding. Absence of the 10 beta-methyl group increased affinity of the androstane compounds for the receptor. The 17 beta-hydroxyl group was also essential for high affinity binding and addition of a 17 alpha-methyl group enhanced binding. Binding of steroids with a delta 4 double bond was consistently less than that of the 5 alpha-reduced steroids. This was true of both the androstene and estrene series. We conclude that human foreskin fibroblasts offer a useful model for in vitro studies characterizing the effects of steroid structural modifications on binding to the human androgen receptor.

Bilayer stabilizing peptides and the inhibition of viral infection: antimeasles activity of carbobenzoxy-Ser-Leu-amide

A number of carbobenzoxy-dipeptide-amides raise the bilayer to hexagonal phase transition temperature of dielaidoylphosphatidylethanolamine (stabilizes the bilayer). The potency of the peptides in stabilizing the bilayer phase is Z-Tyr-Leu-NH2= Z-Gly-Phe-NH2>Z-Ser-Leu-NH2>Z-Gly-Leu-NH2>Z-Gly-Gly-NH2. A linear correlation was found between the respective HPLC retention time parameterk′ for the peptide and the slope of the bilayer stabilization curve determined with model membranes by differential scanning calorimetry. One dipeptide, Z-Ser-Leu-NH2, reduces measles virus cytopathic effect (CPE) in Vero cells. The mechanism by which this peptide reduces the CPE is not known, although some peptides which raise the bilayer to hexagonal phase transition temperature of phospholipids inhibit membrane fusion.

Molecular properties of the androgen receptor in rat ventral prostate.

Results from these studies demonstrate that we have purified a protein from rat prostate cytosol that is similar to the beta-protein (complex II) but different from the alpha-protein (complex I) reported by Liao et al. The purified receptor was different from androgen binding protein (ABP) in that ABP has a faster dissociation rate (6 min), a lower pI value (4.6), and requires higher concentrations of ammonium sulfate for precipitation (40-50%) than the prostatic androgen receptor. It is not likely that we have purified a serum sex-steroid binding protein since no such protein is found in rat serum. This report presents a rapid and efficient procedure for the purification of androgen receptor from rat ventral prostate. However, the present procedure only allowed us to obtain a limited quantity of purified receptor from each preparation. It is obvious that we need to scale up the purification of the receptor in order to study in detail its physicochemical properties and to produce monospecific antibodies against the protein. This work is in progress. In addition, we have demonstrated that two affinity labels can be used to bind covalently to the androgen receptor. Most importantly, these compounds can be used to characterize androgen receptors under both nondenaturing and denaturing conditions and represent useful tools for future work with androgen receptor proteins and androphilic proteins in general.

A β-turn mimic and a thiomethylene dipeptide surrogate employed in the study of cyclic peptide RGD and RCD cell-adhesion inhibitors

During an inflammatory response, leukocytes adhere to the blood-vessel wall and to underlying extracellular matrix-protein fibronectin via noncovalent interaction through two distinct cell-surface integrins, α5β1 and α4β1. An Arg-Gly-Asp tripeptide in the cell-binding domain of fibronectin has been demonstrated to be the major site that mediates cell attachment. Our group has previously disclosed Arg-Gly-Asp- and Arg-Cys-Asp-containing cyclic peptide cell-adhesion inhibitors, which are potentially useful as anti-inflammatory agents, particularly in the treatment of asthma and rheumatoid arthritis. Subsequently, we have incorporated a β-turn dipeptide mimic and a suitably protected Arg-Gly thiomethylene dipeptide surrogate into our lead peptides by solid-phase synthesis. The preparation of the surrogates, and the structure-activity studies of the surrogate-containing peptides are described in this paper. Complete loss of cell-adhesion inhibition activity was observed in the case of incorporation of the conformationally constrained β-turn dipeptide into a cyclic Arg-Cys-Asp peptide inhibitor. Incorporation of an Arg-Gly thiomethylene dipeptide surrogate into cyclic Arg-Gly-Asp and Arg-Cys-Asp cyclic disulfide peptide inhibitors was tolerated, resulting in cyclic pseudopeptide cell-adhesion inhibitors lacking the Arg-Gly peptide amide bond.