IL-8 single-chain homodimers and heterodimers: interactions with chemokine receptors CXCR1, CXCR2, and DARC.

Covalent single-chain dimers of the chemokine interleukin-8 (IL-8) have been designed to mimic the dimeric form of IL-8 in solution and facilitate the production of heterodimer variants of IL-8. Physical studies indicated that use of a simple peptide linker to join two subunits, while allowing receptor binding and activation, led to self-association of the tethered dimers. However, addition of a single disulfide crosslink between the tethered subunits prevented this multimer from forming, yielding a species of dimer molecular weight. Crosslinked single-chain dimers bind to both IL-8 neutrophil receptors CXCR1 and CXCR2 as well as to DARC, as does a double disulfide-linked dimer with no peptide linker. In addition, neutrophil response to these dimers as measured by chemotaxis or beta-glucuronidase release is similar to that elicited by wild-type IL-8, providing evidence that the dissociation of the dimeric species is not required for these biologically relevant activities. Finally, through construction of single-chain heterodimer mutants, we show that only the first subunit's ELR motif is the single-chain variants.

Structure and bacterial receptor activity of a human salivary proline-rich glycoprotein.

Using an overlay technique, we previously showed that the Gram-negative periodontal pathogen Fusobacterium nucleatum binds to a glycoprotein of Mr 89,000 (Prakobphol, A., Murray, P., and Fischer, S.J. (1987) Anal. Biochem. 164, 5-11) in the parotid saliva of some individuals. We now show that deglycosylation of the purified glycoprotein results in loss of receptor activity. Amino acid analysis of the protein core showed predominantly proline, glycine, and glutamic acid/glutamine, a characteristic of proline-rich glycoproteins (PRG). The amino terminus contained repeating sequences of Ser-Gln-Gly-Pro-Pro-Pro-Arg-Pro-Gly-Lys-Pro-Glu-Gly-Pro-Pro-Pro- Gln-Gly that had significant compositional and sequence homology to that encoded by exon 3 of the PRB3 gene. We analyzed the PRG oligosaccharides by a combination of mass spectrometry techniques and nuclear magnetic resonance spectroscopy. Twenty-seven highly fucosylated structures were identified. The most abundant was as follows (where Fuc is fucose). (formula; see text) To understand the structural basis of F. nucleatum binding, we screened glycolipids and neoglycolipids carrying carbohydrate structures related to those of the PRG for receptor activity; components with unsubstituted terminal lactosamine residues best supported adherence. Neoglycolipids constructed from PRG oligosaccharides were also receptors. Treatment with beta-galactosidase, but not alpha-fucosidase, abolished binding, suggesting that unsubstituted lactosamine units, including the 6-antenna of the major oligosaccharide, mediate F. nucleatum adherence.

Phosphorylation site of NtrC, a protein phosphatase whose covalent intermediate activates transcription.

The NtrC transcription factor is a member of a family of homologous prokaryotic regulatory proteins that participate in the transduction of extracellular and nutritional signals. It has been demonstrated that the phosphate group from a histidine residue of the phosphorylated NtrB protein autokinase is transferred to the NtrC protein. Phosphorylation of the NtrC protein is transient and activates its transcriptional enhancement activity. We have investigated the site of phosphorylation of the Salmonella typhimurium NtrC protein and find that it is an aspartate residue (Asp-54) that is found within a sequence conserved in all of the members of the family of regulatory proteins. We propose that this phosphorylation is an NtrC protein histidine phosphatase catalytic intermediate. This conclusion suggests that the NtrC family should be viewed not as kinase substrates but as enzymes that can catalyze the hydrolysis of their activated forms in a concentration-independent fashion. They are similar in this sense to eukaryotic signal-transducing GTPases.

Structure of the oligosaccharide portion of human hepatitis B surface antigen.

The hepatitis B surface antigen, which constitutes the currently available vaccine, is the empty envelope of the hepatitis B virus. We investigated the carbohydrate structures of the envelope glycoproteins. The intact oligosaccharides were enzymatically released from the coat glycoproteins using peptide-N4-(N-acetyl-beta-glucosaminyl) asparagine amidase F and isolated by gel permeation chromatography. Cesium ion liquid secondary ion mass spectra of the intact, underivatized oligosaccharides showed molecular weights of 1932, 2078, and 2223. The mixture included partially and totally sialylated structures, a fraction (approximately 8%) of which were substituted with a single terminal fucose residue; no desialylated oligosaccharides were detected. The reducing termini of the oligomers were derivatized by reduction of the Schiff base formed using p-aminobenzoic acid ethyl ester, and fragmentation patterns identical to those produced from standard biantennary complex oligosaccharides were obtained. Methylation linkage analysis of the oligosaccharides showed that the carbohydrate composition and the mannose branching patterns also resembled those of a biantennary oligosaccharide. The results of this study indicate that glycosylation of the hepatitis B surface antigen, which takes place in the liver, is typical of other serum glycoproteins made in the liver; and this analytical strategy, including cesium ion liquid secondary ion mass spectrometry, is an effective approach for the structural analysis of complex carbohydrates available in only the 1-10 micrograms sample size range.

Identification of the site of phosphorylation of the chemotaxis response regulator protein, CheY.

The protein (Escherichia coli CheY) that controls the direction of flagellar rotation during bacterial chemotaxis has been shown to be phosphorylated on the aspartate 57 residue. The residue phosphorylated is present within a conserved sequence in every member of a family of bacterial regulatory proteins. The phosphorylation is transient, with a much shorter half-life than that expected of a simple acyl phosphate intermediate, indicating that the sequence and conformation of the protein is designed to achieve a rapid hydrolysis. The CheY-phosphate linkage can be reductively cleaved by sodium borohydride. High-performance tandem mass-spectrometric analysis of proteolytic peptides derived from [3H]borohydride-reduced phosphorylated CheY protein was used to identify the position of phosphorylation. Mutants with altered aspartate 57 exhibited no chemotaxis. When aspartate 13, another conserved residue, was changed, greatly reduced chemotaxis was observed, suggesting an important role for aspartate 13. The rate-determining step of chemotactic signaling is governed by the kinetics of formation and hydrolysis of the CheY protein phosphoaspartate bond. The CheY protein apparently functions as a protein phosphatase that possesses a transient covalent intermediate. Transient phosphorylation of an aspartate residue is an effective mechanism for producing a biochemical signal with a short concentration-independent half-life. The duration of the signal can be controlled by small structural elements within the phosphorylated protein.

A new Saccharomyces cerevisiae mnn mutant N-linked oligosaccharide structure.

We find that the N-linked Man8GlcNAc2- core oligosaccharide of Saccharomyces cerevisiae mnn mutant mannoproteins is enlarged by the addition of the outer chain to the alpha 1----3-linked mannose in the side chain that is attached to the beta 1----4-linked mannose rather than by addition to the terminal alpha 1----6-linked mannose. This conclusion is derived from structural studies on a phosphorylated oligosaccharide fraction and from mass spectral fragment analysis of neutral core oligosaccharides.

Structural characterization of intact, branched oligosaccharides by high performance liquid chromatography and liquid secondary ion mass spectrometry.

We report results of a mass-spectrometric-based strategy for determining the detailed structural features of N-linked oligosaccharides from glycoproteins. The method was used to characterize a series of intact, high mannose oligosaccharides isolated from human immunoglobulin M (IgM). The IgM was purified from a patient with Waldenstrom's macroglobulinemia. The strategy included releasing the oligosaccharides by digestion of the purified glycoprotein with endoglycosidase H, separating the released oligosaccharides by high resolution gel filtration, and derivatizing the resulting reducing termini with the uv-absorbing moiety, ethyl p-aminobenzoate. This particular derivative facilitates HPLC detection and provides centers for protonation and deprotonation enhancing liquid secondary ion mass spectra. Positive and negative ion spectra contained molecular species of similar abundance. However, fragment ion peaks yielding sequence information were significantly more prominent in the negative ion mass spectra. Furthermore, it was obvious that the fragmentation patterns differed substantially for linear and branched oligomers. For linear oligosaccharides, a smooth envelope of fragment ions was observed; from low to high mass there was an ordered decrease in ion abundance from both the reducing and nonreducing termini. This pattern of fragment ions was not observed for branched oligosaccharides since in these cases fragments at certain masses could not arise by single bond cleavages. Therefore, these fragments were either significantly reduced in abundance or absent as compared with identical fragments formed from linear molecules. Importantly, 200 pmol of an oligosaccharide could be derivatized, separated, and detected by mass spectrometry, allowing identification of previously unreported minor components of the IgM oligosaccharides. Therefore, this experimental strategy is particularly useful for the purification and detailed structural characterization of low abundance oligosaccharides isolated from heterogeneous biological samples.

Exchanging interleukin-8 and melanoma growth-stimulating activity receptor binding specificities.

Interleukin-8 (IL-8), a CXC chemokine, is known to bring about chemotaxis and activation of neutrophils through high affinity binding to at least two distinct receptors, receptor-A and receptor-B. The IL-8 homolog melanoma growth stimulating activity (MGSA) is also active toward neutrophils. In contrast to IL-8, MGSA binds receptor-B with high affinity and binds receptor-A with approximately 400-fold lower affinity. Using the structure of IL-8 (Clore et al.(1990) Biochemistry, 29, 1689-1696; Baldwin et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 502-506) and the NMR-determined structure of MGSA (Fairbrother et al. (1994) J. Mol. Biol. 242, 252-270), we designed variants of both IL-8 and MGSA to investigate the basis of specificity for binding of these chemokines to the IL-8 receptors. The most outstanding structural difference between IL-8 and MGSA lies in the loop preceding the first beta-strand. When the corresponding (shorter) loop from MGSA was swapped into IL-8, both receptor-A and receptor-B binding affinities were significantly (>300-fold) reduced. However, with additional mutations that affect packing interactions, an IL-8 variant specific for receptor-B binding was produced. Conversely, when the same loop from IL-8 was swapped into MGSA, receptor-B binding was maintained with only a approximately 30-fold reduction in receptor-A affinity. Again, mutations affecting packing of the loop yielded a MGSA variant with high affinity for both receptors, like IL-8. Finally, we show, through point mutations in a monomeric IL-8 framework, that individual side chain substitutions can affect receptor specificity.

Confirmation by mass spectrometry of a trisulfide variant in methionyl human growth hormone biosynthesized in Escherichia coli.

A sulfur-containing compound found in acid hydrolysates of proteins was identified 30 years ago as a trisulfide: bis-(2-amino-2-carboxyethyl) trisulfide (cysteine2S3). At that time, studies concerning the chemistry of sulfur-transferring enzyme systems suggested that cysteine2S3 also existed in biological systems. Two decades later, a cystine trisulfide structure was postulated in the regulator protein molecule for the activation of delta-aminolevulinate synthetase. Recently, a trisulfide bond was reported to occur in the minor loop disulfide at Cys182-Cys189 in human growth hormone. We have detected a trisulfide structure in methionyl human growth hormone in the major loop disulfide Cys53-Cys165. The development of mass spectral analyses of high molecular weight molecules, such as proteins, led to the eventual identification of the modification. A tandem mass spectral analysis on a Sciex electrospray instrument localized an addition of 32 Da to the Cys53-Cys165 fragment. Elemental composition was determined by accurate mass measurement obtained by peak matching to a synthetic peptide and established that an extra sulfur atom was involved.

Structure determination of O-linked glycopeptides by tandem mass spectrometry.

A new method for characterizing O-linked glycopeptides without chemical degradation is presented. Collision-induced dissociation (CID) analysis of intact O-linked glycopeptides containing mono- and disaccharides was performed. For glycopeptides containing one hexose unit, both the peptide sequence and the site of attachment of the sugar moiety were obtained from a single high-energy CID spectrum. However, in a glycopeptide bearing multiple sugar residues per site, the CID spectrum was dominated by fragments resulting from cleavages of the carbohydrate substituents and the gas-phase deglycosylated peptide, thus obviating the concomitant observation of peptide sequence ions. Hence, information on the structures of the carbohydrate substituents was obtained, but not on the sites of attachment of these residues to the peptide. Subsequent CID analysis of the gas-phase deglycosylated peptide ion can be used to obtain the sequence of the peptide backbone from the same sample. This method holds promise for simultaneously determining the carbohydrate structure and the peptide sequence of intact O-linked glycopeptides without chemical degradation.

A His19 to Ala mutant of melanoma growth-stimulating activity is a partial antagonist of the CXCR2 receptor.

Melanoma growth stimulating activity (MGSA) and IL-8 are related chemokines that are potent chemoattractants and activators of neutrophils both in vitro and in vivo. Increasing evidence suggests that these molecules play an important role in inflammation; thus, antagonists of their action could be useful therapeutically as antiinflammatory agents. We have generated an MGSA mutant, H19A, that shows a dissociation between receptor binding and biologic activity. The biologic activity of the H19A mutant is between 133-fold and 282-fold less potent than that of wild-type MGSA measured by three independent assays of neutrophil function, i.e., elastase release chemotaxis and the up-regulation of CD18. In addition, pretreatment of cells with the H19A mutant inhibited the ability of MGSA to induce elastase release and chemotaxis and to increase intracellular calcium. However, competition binding studies in cells transfected with the CXCR2 receptor and in neutrophils demonstrate that the receptor affinity of the H19A mutant is only 13-fold less than that of wild-type MGSA. These studies suggest that the mutant MGSA is defective in activating signaling through the receptor and indicate that binding to the receptor is not sufficient to activate a biologic response. The dissociation between receptor binding and activation for this mutant suggests that it should be possible to design antagonists of MGSA that may be of clinical utility.

A mutant of melanoma growth stimulating activity does not activate neutrophils but blocks erythrocyte invasion by malaria.

Alanine scanning mutagenesis of the charged amino acids of melanoma growth stimulating activity (MGSA) was used to identify specific residues that are involved in binding to the human erythrocyte Duffy antigen/chemokine receptor (DARC) and to the type B interleukin-8 receptor (IL-8RB) on neutrophils. Receptor binding and biological studies with the alanine scan mutants of MGSA demonstrate that MGSA binds to DARC and the IL-8RB through distinct binding regions. One of the MGSA mutants, E6A, binds to human erythrocytes and is able to inhibit malaria invasion as efficiently as wild type MGSA but has a severely reduced ability to bind to or signal through the IL-8RB. Mutant chemokines like E6A could prove to be useful therapeutically for the design of receptor blocking drugs that inhibit erythrocyte invasion by Plasmodium vivax malaria.