Effect of nanomolar concentrations of sodium dodecyl sulfate, a catalytic inductor of alpha-helices, on human calcitonin incorporation and channel formation in planar lipid membranes

High-level expression and characterization of the Bacillus subtilis subsp. subtilis str. BSP1 YwaD aminopeptidase in Pichia pastoris

Aminopeptidases are widely used for creating protein hydrolysates and peptide sequencing. The ywaD gene from a new Bacillus isolate, named Bacillus subtilis subsp. subtilis str. BSP1, was cloned into the yeast expression vector pHBM905A and expressed and secreted by Pichia pastoris strain GS115. The deduced amino acid sequence of the aminopeptidase encoded by the ywaD gene shared up to 98% identity with aminopeptidases from B. subtilis strains 168 and zj016. The yield (3.81 g/l) and specific activity (788 U/mg) of recombinant YwaD in high-density fermentation were extremely high. And 829.83 mg of the purified enzyme (4089.72 U/mg) were harvested. YwaD was glycosylated, and its activity decreased after deglycosylation, which was similar to that of the aminopeptidase from B. subtilis strain zj016. YwaD was most active toward l-arginine-4-nitroanilide. Moreover, it exhibited high resistance to carbamide, which was not true for aminopeptidases from B. subtilis strains 168 and zj016, which could simplify the purification of YwaD. Moreover, the expression and parts of characterization of the aminopeptidase from B. subtilis strain 168 in Pichia pastoris were added as supplementary material. The sequence and other characteristics of YwaD were compared with those of aminopeptidases from B. subtilis strains 168 and zj016, and they will provide a solid foundation for further research on the influence of amino acid mutations on the function of aminopeptidases.

Biophysical characterization of a recombinant aminopeptidase II from the thermophilic bacterium Bacillus stearothermophilus

In the present study, the biophysical properties of His6-tagged Bacillus stearothermophilus aminopeptidase II (His6-tagged BsAmpII) are characterized in detail by gel-filtration, analytical ultracentrifugation, and various spectroscopic techniques. Using size-exclusion chromatography and analytical ultracentrifugation, we demonstrate that His6-tagged BsAmpII exists predominantly as a dimer in solution. The enzyme is active and stable at pHs ranging from 6.5 to 8.5. Far-UV circular dichroism analysis reveals that the secondary structures of His6-tagged BsAmpII are significantly altered in the presence of SDS, whereas the presence of 5-10% acetone and ethanol was harmless to the folding of the enzyme. Thermal unfolding of His6-tagged BsAmpII was found to be irreversible and led to the formation of aggregates. The native enzyme started to unfold beyond 0.6 M guanidine hydrochloride and had a midpoint of denaturation at 1.34 M. This protein remained active at concentrations of urea below 2.7 M but experienced an irreversible unfolding by >5 M denaturant. Taken together, this work lays a foundation for potential biotechnological applications of His6-tagged BsAmpII.

Investigating the effects of sodium dodecyl sulfate on the aggregative behavior of hen egg-white lysozyme at acidic pH

The research presented here is aimed at examining the effects of sodium dodecyl sulfate on the aggregative behavior of hen egg-white lysozyme at pH 2.0. Through various spectroscopic techniques, dynamic light scattering, and electron microscopy, we first demonstrated that SDS exhibited a biphasic effect on lysozyme fibrillation. The presence of SDS at higher concentrations (e.g., 0.25, 5.00, or 20.00 mM SDS) was found to suppress fibril formation of lysozyme whereas fibrillogenic lysozyme-SDS ensemble containing beta-sheet-rich conformation was observed upon the addition of lower concentrations of SDS (e.g., 0.00, 0.06, or 0.1mM SDS). Next, our equilibrium urea-unfolding data revealed that lysozyme samples with higher SDS concentrations showed superior thermodynamic stabilities over the ones with no or lower levels of SDS. Finally, the correlation between SDS concentration and lysozyme aggregative/fibrillogenic propensity and the underlying interacting mechanism were further explored using surface tensiometry and isothermal titration calorimetry. We believe the outcome from this work may not only help decipher the molecular mechanism of amyloid fibrillation, but also shed light on a rational design of potential therapeutic strategies for amyloid pathology.

Dual behavior of sodium dodecyl sulfate as enhancer or suppressor of insulin aggregation and chaperone-like activity of camel alphaS(1)-casein

Sodium dodecyl sulfate (SDS) at low concentrations considerably enhanced insulin aggregation and reduced the chaperone-like activity of purified camel alphaS(1)-casein (alphaS(1)-CN). These observed changes were the result of repulsive electrostatic interactions between both negative charged head groups of SDS and alphaS(1)-CN, and the net negative charge of insulin molecules, resulting in the greater exposure of hydrophobic patches of insulin and its enhanced aggregation. In contrast, enhanced hydrophobic interactions were primarily responsible for the conformational changes observed in insulin and alphaS(1)-CN at high SDS concentrations, resulting in increased binding of SDS and alphaS(1)-CN to insulin and its reduced aggregation.

Conformation and lipid binding of a C-terminal (198-243) peptide of human apolipoprotein A-I

Human apolipoprotein A-I (apoA-I) is the principle apolipoprotein of high-density lipoproteins that are critically involved in reverse cholesterol transport. The intrinsically flexibility of apoA-I has hindered studies of the structural and functional details of the protein. Our strategy is to study peptide models representing different regions of apoA-I. Our previous report on [1-44]apoA-I demonstrated that this N-terminal region is unstructured and folds into approximately 60% alpha-helix with a moderate lipid binding affinity. We now present details of the conformation and lipid interaction of a C-terminal 46-residue peptide, [198-243]apoA-I, encompassing putative helix repeats 10 and 9 and the second half of repeat 8 from the C-terminus of apoA-I. Far-ultraviolet circular dichroism spectra show that [198-243]apoA-I is also unfolded in aqueous solution. However, self-association induces approximately 50% alpha-helix in the peptide. The self-associated peptide exists mainly as a tetramer, as determined by native electrophoresis, cross-linking with glutaraldehyde, and unfolding data from circular dichroism (CD) and differential scanning calorimetry (DSC). In the presence of a number of lipid-mimicking detergents, above their CMC, approximately 60% alpha-helix was induced in the peptide. In contrast, SDS, an anionic lipid-mimicking detergent, induced helical folding in the peptide at a concentration of approximately 0.003% (approximately 100 microM), approximately 70-fold below its typical CMC (0.17-0.23% or 6-8 mM). Both monomeric and tetrameric peptide can solubilize dimyristoylphosphatidylcholine (DMPC) liposomes and fold into approximately 60% alpha-helix. Fractionation by density gradient ultracentrifugation and visualization by negative staining electromicroscopy demonstrated that the peptide binds to DMPC with a high affinity to form at least two sizes of relatively homogeneous discoidal HDL-like particles depending on the initial lipid:peptide ratio. The characteristics (lipid:peptide weight ratio, diameter, and density) of both complexes are similar to those of plasma A-I/DMPC complexes formed under similar conditions: small discoidal complexes (approximately 3:1 weight ratio, approximately 110 A, and approximately 1.10 g/cm3) formed at an initial 1:1 weight ratio and larger discoidal complexes (approximately 4.6:1 weight ratio, approximately 165 A, and approximately 1.085 g/cm3) formed at initial 4:1 weight ratio. The cross-linking data for the peptide on the complexes of two sizes is consistent with the calculated peptide numbers per particle. Compared to the approximately 100 A disk-like complex formed by the N-terminal peptide in which helical structure was insufficient to cover the disk edge by a single belt, the compositions of these two types of complexes formed by the C-terminal peptide are more consistent with a "double belt" model, similar to that proposed for full-length apoA-I. Thus, our data provide direct evidence that this C-terminal region of apoA-I is responsible for the self-association of apoA-I, and this C-terminal peptide model can mimic the interaction with the phospholipid of plasma apoA-I to form two sizes of homogeneous discoidal complexes and thus may be responsible for apoA-I function in the formation and maintenance of HDL subspecies in plasma.

Acetyl-[Asn30,Tyr32]-calcitonin fragment 8-32 forms channels in phospholipid planar lipid membranes

The N-terminally truncated derivative of salmon calcitonin (sCt) (acetyl-[Asn(30),Tyr(32)]-calcitonin fragment 8-32) (AC 187) lacks hormonal activity and is a potent and selective antagonist of the hormone and amylin receptor. It was investigated for its capability to interact and form channels in palmitoleoylphosphatidylcholine:dioleoylphosphatidylglycerol planar lipid membranes. Interestingly, AC 187 exhibits channel activity, whose parameters, i.e., central conductance (Lambda (c)), occurrence (number of channels/min), voltage-dependence and lifetime, are similar to those found for sCt although, in the same experimental conditions, it takes longer to incorporate into the membrane than sCt. This channel activity can be modulated by changing either the holding potential or the pH of the medium, or by adding picomolar concentrations of SDS. One evident difference between the two peptides is that sCt is unselective (1.03) while AC 187 displays a cationic selectivity (P (K) (+)/P (Cl) (-) = 2.7) at pH 7, increasing to 3.87 when the pH drops to 3.8. The present findings indicate that the 1-7 disulfide bridge is sufficient but not necessary for membrane interaction, in accordance with the observation reported on the interaction with membrane receptors. Furthermore, the remarkable pH dependence of the cationic channel could be taken into consideration for full biotechnological study.

Effect of eel calcitonin glycosylation on incorporation and channel formation in planar phospholipid membranes

Previous studies have shown that different calcitonins interact with planar lipid membranes to form ion channels. In this study, glycosylation of eel calcitonin (eCt) at different positions (Ct3-GlcNAc, Ct14-GlcNAc, Ct20-GlcNAc, Ct26-GlcNAc) is shown to preserve molecular structure and slightly change the energy of incorporation and channel formation in planar lipid bilayers made up of palmitoyl-oleoyl-phosphatidylcholine:dioleoyl phosphatidyl-glycerol (85:15, w:w). The voltage needed to form channels decreased as the attached carbohydrate moved toward the C-terminal (eCt = Ct3-GlcNAc > Ct14-GlcNAc = Ct20-GlcNAc > Ct26-GlcNAc). Interestingly, all the Cts tested maintain the characteristic voltage-conductance dependence found for other Cts, the only channel properties modified concern ion selectivity, that shift toward anion selectivity (eCt = 0.97, Ct3-GlcNAc = 0.49, Ct14-GlcNAc = 0.41, Ct20-GlcNAc = 0.36, Ct26-GlcNAc = 0.47). These aspects would be useful in managing peptide properties for biotechnological and therapeutic applications considering the physiological nature of this peptide.

Transmembrane glycine zippers: physiological and pathological roles in membrane proteins

We have observed a common sequence motif in membrane proteins, which we call a glycine zipper. Glycine zipper motifs are strongly overrepresented and conserved in membrane protein sequences, and mutations in glycine zipper motifs are deleterious to function in many cases. The glycine zipper has a significant structural impact, engendering a strong driving force for right-handed packing against a neighboring helix. Thus, the presence of a glycine zipper motif leads directly to testable structural hypotheses, particularly for a subclass of glycine zipper proteins that form channels. For example, we suggest that the membrane pores formed by the amyloid-beta peptide in vitro are constructed by glycine zipper packing and find that mutations in the glycine zipper motif block channel formation. Our findings highlight an important structural motif in a wide variety of normal and pathological processes.