A comparison of the dodecyl sulfate-induced precipitation of the myelin basic protein with other water-soluble proteins

Mucosal adhesion properties of the probiotic Lactobacillus rhamnosus GG SpaCBA and SpaFED pilin subunits

Lactobacillus rhamnosus GG is a well-established Gram-positive probiotic strain, whose health-benefiting properties are dependent in part on prolonged residence in the gastrointestinal tract and are likely dictated by adherence to the intestinal mucosa. Previously, we identified two pilus gene clusters (spaCBA and spaFED) in the genome of this probiotic bacterium, each of which contained the predicted genes for three pilin subunits and a single sortase. We also confirmed the presence of SpaCBA pili on the cell surface and attributed an intestinal mucus-binding capacity to one of the pilin subunits (SpaC). Here, we report cloning of the remaining pilin genes (spaA, spaB, spaD, spaE, and spaF) in Escherichia coli, production and purification of the recombinant proteins, and assessment of the adherence of these proteins to human intestinal mucus. Our findings indicate that the SpaB and SpaF pilin subunits also exhibit substantial binding to mucus, which can be inhibited competitively in a dose-related manner. Moreover, the binding between the SpaB pilin subunit and the mucosal substrate appears to operate through electrostatic contacts and is not related to a recognized mucus-binding domain. We conclude from these results that it is conceivable that two pilin subunits (SpaB and SpaC) in the SpaCBA pilus fiber play a role in binding to intestinal mucus, but for the uncharacterized and putative SpaFED pilus fiber only a single pilin subunit (SpaF) is potentially responsible for adhesion to mucus.

Interaction of the 18.5-kD isoform of myelin basic protein with Ca2+ -calmodulin: effects of deimination assessed by intrinsic Trp fluorescence spectroscopy, dynamic light scattering, and circular dichroism

The effects of deimination (conversion of arginyl to citrullinyl residues) of myelin basic protein (MBP) on its binding to calmodulin (CaM) have been examined. Four species of MBP were investigated: unmodified recombinant murine MBP (rmMBP-Cit(0)), an engineered protein with six quasi-citrullinyl (i.e., glutaminyl) residues per molecule (rmMBP-qCit(6)), human component C1 (hMBP-Cit(0)), and human component C8 (hMBP-Cit(6)), both obtained from a patient with multiple sclerosis (MS). Both rmMBP-Cit(0) and hMBP-Cit(0) bound CaM in a Ca(2+)-dependent manner and primarily in a 1:1 stoichiometry, which was verified by dynamic light scattering. Circular dichroic spectroscopy was unable to detect any changes in secondary structure in MBP upon CaM-binding. Inherent Trp fluorescence spectroscopy and a single-site binding model were used to determine the dissociation constants: K(d) = 144 +/- 76 nM for rmMBP-Cit(0), and K(d) = 42 +/- 15 nM for hMBP-Cit(0). For rmMBP-qCit(6) and hMBP-Cit(6), the changes in fluorescence were suggestive of a two-site interaction, although the dissociation constants could not be accurately determined. These results can be explained by a local conformational change induced in MBP by deimination, exposing a second binding site with a weaker association with CaM, or by the existence of several conformers of deiminated MBP. Titration with the collisional quencher acrylamide, and steady-state and lifetime measurements of the fluorescence at 340 nm, showed both dynamic and static components to the quenching, and differences between the unmodified and deiminated proteins that were also consistent with a local conformational change due to deimination.

Terminal deletion mutants of myelin basic protein: new insights into self-association and phospholipid interactions

The 18.5kDa isoform of myelin basic protein (MBP) has strong and probably specific interactions with phosphoinositides that are of interest regarding this protein's function, and in effecting its two-dimensional crystallization for structural determination. We have designed and constructed truncation mutants of recombinant 18.5kDa murine myelin basic protein (rmMBP) lacking either the N- or C-terminal third, i.e. rmMBPDeltaN and rmMBPDeltaC, respectively. Both variants rmMBPDeltaC and rmMBPDeltaN generally had a reduced ability to aggregate lipid vesicles, compared to the whole protein, especially at lower protein/lipid ratios. Lipid vesicle cosedimentation showed that both truncated variants exhibited altered binding with phosphatidylinositol (PI). Incubation of these proteins under monolayers comprising PI and a nickel-chelating lipid yielded crystalline arrays of rmMBPDeltaC (but not rmMBPDeltaN) in the absence of high salt or osmolytes, which are required for crystallization of whole protein. This result suggests that the C-terminal segment of MBP is a significant source of conformational heterogeneity, and its removal will facilitate future planar or three-dimensional crystallization attempts. Incubation of rmMBPDeltaN and rmMBPDeltaC under monolayers comprising phosphatidylinositol-4-phosphate and a nickel-chelating lipid yielded tubular structures of opposite chirality, suggesting a synergistic effect of both termini of MBP in organizing myelin lipids.

Selective precipitation of interleukin-4 using hydrophobic ion pairing: a method for improved analysis of proteins formulated with large excesses of human serum albumin

In order to ensure the stability of protein pharmaceuticals, human serum albumin (HSA) is often added as an excipient, frequently in large excess. This makes chromatographic analysis of the stability of the active protein difficult. In the case of interleukin-4 (IL-4), separation from HSA can be achieved to some degree by size exclusion chromatography, but some HSA co-elutes with the IL-4. Hydrophobic ion pairing provides a method for selective precipitation of IL-4 from HSA. Hydrophobic ion pairing involves the electrostatic interaction of ionic detergents with oppositely charged polypeptides. Even when HSA is present in fifty-fold excess (w/w), the resulting precipitate contains greater than 70% of the IL-4. Selective precipitation with SDS produces enhancements in IL-4 over HSA of more than 2000-fold. This approach permits subsequent facile analysis of IL-4 by conventional reverse phase HPLC.

The basic protein of CNS myelin: its structure and ligand binding

Consideration of the evidence presented in this review leads to the following conclusions: (a) Isolated MBP in aqueous solution has little ordered secondary or tertiary structure. (b) In this state, the protein can associate with a wide range of hydrophobic and amphiphilic compounds, these interactions involving limited sections of the protein. (c) The strength of binding to bilayers and the accompanying conformational changes in the protein are greatest for systems containing acidic lipids, presumably because of the involvement of ionic interactions. (d) When bound to bilayers of acidic lipids, MBP will have substantially more ordered secondary structure than it manifests in aqueous solution, and it is likely to be oligomeric (possibly hexameric). (e) MBP does affect the organization of lipid aggregates. It influences strongly the separation of bilayers in multilayers of purified lipids, and at present this must be viewed as its prime role within myelin. The greatest impediment to our understanding of MBP is the lack of an assayable biological activity. In contrast to the situation with enzymes, for example, we have no functional test for changes in protein structure or changes accompanying interactions with other molecules. Current evidence suggests that the protein has a structural role within myelin and that its own three-dimensional structure is strongly dependent on the molecules with which it is associated. If this picture is correct, studies of the isolated protein or of the protein in reconstituted lipid systems may yield, at best, a rough guide to the structure within its biological environment. Further clarification of the structure and function of MBP may have to await development of more powerful techniques for studying proteins bound to large molecular aggregates, such as lipid bilayers. The paucity of generally applicable methods is reflected in the fact that even low resolution structures are known for only a handful of intrinsic membrane proteins, and even more limited information exists for proteins associated with membrane surfaces. However, the increasing use of a combination of electron microscopy and diffraction on two-dimensional arrays of proteins formed on lipid bilayers (Henderson et al., 1990) offers the hope that it may not be too long before it will be possible to study at moderate resolution the three-dimensional structure of MBP bound to a lipid membrane.