Characterization of a stable intermediate trapped during reversible refolding of Bacillus subtilis alpha-amylase

Cloning, overexpression, and structural characterization of a novel archaeal thermostable neopullulanase from Desulfurococcus mucosus DSM 2162

The main purpose of the present study is to introduce the biochemical characteristics of the industrial valuable thermostable pullulan degrading enzyme from Desulfurococcus mucosus DSM2162. Recombinant protein was purified by a combination of thermal treatment and affinity chromatography, with a yield of 15.94% and 7.69-fold purity. Purified enzyme showed the molecular mass of 55,787 Da with optimum activity at 70 °C and a broad range of pH (5.0-9.0) with kcat of 2150 min^-1 and Km of 6.55 mg.mL^-1, when using starch as substrate. The enzyme activity assay on various polysaccharide substrates revealed the substrate preference of pullulan > amylopectin > β cyclodextrin > starch > glycogen; therefore, it classified as a neopullulanase. The neopullulanase structural analysis by spectrofluorometer, FT-IR, and circular dichroism spectroscopy indicated the corporation of α-helix (47.3%) and β-sheet (31.6%) in its secondary structure. The melting temperature and specific heat capacity calculations using differential scanning calorimetry confirmed its extreme thermal stability. Further, salt-elevated concentrations resulted in oligomeric state dominancy without any significant influence on the starch-degrading ability. The newly cloned archaeal neopullulanase was with broad activity on polysaccharide substrates, with thermal and salt stability. Thus, the Desulfurococcus mucosus DSM2162 neopullulanase can be introduced as a good candidate to be used in carbohydrate industry.

Lactococcus lactis As a Versatile Vehicle for Tolerogenic Immunotherapy

Genetically modified Lactococcus lactis bacteria have been engineered as a tool to deliver bioactive proteins to mucosal tissues as a means to exert both local and systemic effects. They have an excellent safety profile, the result of years of human consumption in the food industry, as well as a lack of toxicity and immunogenicity. Also, containment strategies have been developed to promote further application as clinical protein-based therapeutics. Here, we review technological advancements made to enhanced the potential of L. lactis as live biofactories and discuss some examples of tolerogenic immunotherapies mediated by mucosal drug delivery via L. lactis. Additionally, we highlight their use to induce mucosal tolerance by targeted autoantigen delivery to the intestine as an approach to reverse autoimmune type 1 diabetes.

Protein transport across the cell wall of monoderm Gram-positive bacteria

In monoderm (single-membrane) Gram-positive bacteria, the majority of secreted proteins are first translocated across the cytoplasmic membrane into the inner wall zone. For a subset of these proteins, final destination is within the cell envelope as either membrane-anchored or cell wall-anchored proteins, whereas another subset of proteins is destined to be transported across the cell wall into the extracellular milieu. Although the cell wall is a porous structure, there is evidence that, for some proteins, transport is a regulated process. This review aims at describing what is known about the mechanisms that regulate the transport of proteins across the cell wall of monoderm Gram-positive bacteria.

Structure and enzyme properties of Zabrotes subfasciatus alpha-amylase

Digestive alpha-amylases play an essential role in insect carbohydrate metabolism. These enzymes belong to an endo-type group. They catalyse starch hydrolysis, and are involved in energy production. Larvae of Zabrotes subfasciatus, the Mexican bean weevil, are able to infest stored common beans Phaseolus vulgaris, causing severe crop losses in Latin America and Africa. Their alpha-amylase (ZSA) is a well-studied but not completely understood enzyme, having specific characteristics when compared to other insect alpha-amylases. This report provides more knowledge about its chemical nature, including a description of its optimum pH (6.0 to 7.0) and temperature (20-30 degrees C). Furthermore, ion effects on ZSA activity were also determined, showing that three divalent ions (Mn2+, Ca2+, and Ba2+) were able to enhance starch hydrolysis. Fe2+ appeared to decrease alpha-amylase activity by half. ZSA kinetic parameters were also determined and compared to other insect alpha-amylases. A three-dimensional model is proposed in order to indicate probable residues involved in catalysis (Asp204, Glu240, and Asp305) as well other important residues related to starch binding (His118, Ala206, Lys207, and His304).

Protein secretion in Lactococcus lactis : an efficient way to increase the overall heterologous protein production

Lactococcus lactis, the model lactic acid bacterium (LAB), is a food grade and well-characterized Gram positive bacterium. It is a good candidate for heterologous protein delivery in foodstuff or in the digestive tract. L. lactis can also be used as a protein producer in fermentor. Many heterologous proteins have already been produced in L. lactis but only few reports allow comparing production yields for a given protein either produced intracellularly or secreted in the medium. Here, we review several works evaluating the influence of the localization on the production yields of several heterologous proteins produced in L. lactis. The questions of size limits, conformation, and proteolysis are addressed and discussed with regard to protein yields. These data show that i) secretion is preferable to cytoplasmic production; ii) secretion enhancement (by signal peptide and propeptide optimization) results in increased production yield; iii) protein conformation rather than protein size can impair secretion and thus alter production yields; and iv) fusion of a stable protein can stabilize labile proteins. The role of intracellular proteolysis on heterologous cytoplasmic proteins and precursors is discussed. The new challenges now are the development of food grade systems and the identification and optimization of host factors affecting heterologous protein production not only in L. lactis, but also in other LAB species.

Bacillus subtilis α-amylase: interactions of a partially folded conformer with small unilamellar vesicles

We studied the interactions between conformers of exocellular alpha-amylase and small unilamellar vesicles (SUV) composed of the major membrane lipids of Bacillus subtilis under physiological conditions of pH, temperature and ionic strength. Using fluorescence spectroscopy, surface plasmon resonance (SPR) and phase separation, we show that the native alpha-amylase has no affinity for the SUV, whereas a partially folded form, displaying structural properties in common with the competent state for secretion, binds to the vesicles (KA approximately 10(5) M(-1)). This association prevented its subsequent folding. The complex was destabilized in the presence of PrsA, a major peripheric lipoprotein of B. subtilis which displays a strong affinity for SUV (KA approximately 1.5x10(8) M(-1)). Vesicles coated with PrsA lost their ability to bind the partially folded conformer. The approach in vitro, in which our aim was to mimic the last stage of alpha-amylase translocation, indicates that PrsA possibly helps, in vivo, the secreted protein to acquire its native conformation by modulating the interaction between the latter and the lipid polar heads on the trans side of the cytoplasmic membrane.

The extracytoplasmic folding factor PrsA is required for protein secretion only in the presence of the cell wall in Bacillus subtilis

Pulse-chase labelling was used to study the role of the cell wall microenvironment in the functioning of Bacillus subtilis PrsA, an extracellular lipoprotein and member of the parvulin family of peptidylprolyl cis/trans-isomerases. It was found that in protoplasts, and thus in the absence of a cell wall matrix, the post-translocational folding, stability and secretion of the AmyQ alpha-amylase were independent of PrsA, in contrast to the strict dependency found in rods. The results indicate that PrsA is dedicated to assisting the folding and stability of exported proteins in the particular microenvironment of the cytoplasmic membrane-cell wall interface, possibly as a chaperone preventing unproductive interactions with the wall. The data also provide evidence for a crucial role of the wall in protein secretion. The presence of the wall directly or indirectly facilitates the release of AmyQ from the cell membrane and affects the rate of the signal peptide processing.

Calcium triggers the refolding of Bacillus subtilis chitosanase

We characterized the reversible folding-unfolding transition of Bacillus subtilis exocellular chitosanase from either thermal or urea denaturation of the protein. The transitions were monitored in each case by intrinsic fluorescence changes and resistance to proteolysis. Unfolding and refolding kinetics and differential scanning calorimetry analysis suggested a two-state equilibrium. The equilibrium between the folded and unfolded states was rapidly displaced towards the folded state in the presence of a low concentration of calcium (2-20 mM). The binding titration curve indicated that chitosanase possesses one weak Ca(2+)-binding site (with an equilibrium affinity constant, K (A), of 0.3x10(3) M(-1)). These results support the hypothesis that this metal ion, which is accumulated in the cell wall environment of B. subtilis, is an effector that influences folding and stability of newly translocated proteins.

Chemical modification of bacterial alpha-amylases: changes in tertiary structures and the effect of additional calcium

A comparative study was performed on the effect of calcium on native and chemically modified forms of mesophilic and thermophilic alpha-amylases. Circular dichroism (CD) and irreversible thermoinactivation studies were carried out in the absence and presence of 10 mM calcium. From the CD experiments, changes in the tertiary structure of these enzymes, brought about by modification, were concluded. Furthermore, these changes were found to be influenced by the presence of calcium. Sorbitol was very effective in affording protection against irreversible thermoinactivation of native and modified forms of the enzymes, both in the absence and presence of calcium. Results are discussed in terms of the usefulness of this new approach involving a combination of medium and chemical modification for protein stabilization and enhancement of catalytic potential.

Anionic polymers of Bacillus subtilis cell wall modulate the folding rate of secreted proteins

In order to characterize the dynamics of the interaction between the emergent membrane translocated exoprotein and the components of Bacillus subtilis cell wall, we examined the kinetics of the in vitro refolding of levansucrase and alpha-amylase, at pH 7 and 37 degrees C, in the presence of polyphosphates (polyP) of various chain lengths (2</=n</=65). These soluble anionic polymers are considered here to mimic the role of teichoic acids. Even in the absence of calcium, levansucrase rapidly refolded in the presence of polyP of n>/=16. In contrast, polyP modulate indirectly the rate of alpha-amylase refolding via their affinity for calcium. These differential effects might explain that the rate of the cell wall translocation of alpha-amylase secretion was found to be half that of levansucrase.

Bacillus subtilis alpha-amylase: the rate limiting step of secretion is growth phase-independent

When Bacillus subtilis alpha-amylase was expressed under the control of sacR in a degU32(Hy) strain, the production of exoenzyme occurred during both the exponential and stationary phases of growth. In each phase, pulse-chase experiments showed that the rate-limiting step of the secretion process was the release of the processed form of the protein in each physiological context. The rate of this event was slightly slower (t(1/2) = 3.2 min) during the stationary phase than during the exponential phase (t(1/2) = 2 min). The effectors which possibly control the efficiency of the release stage, the level of PrsA or the calcium binding properties of the cell wall, remained unchanged throughout growth phases.

Differential dependence of levansucrase and alpha-amylase secretion on SecA (Div) during the exponential phase of growth of Bacillus subtilis

SecA, the translocation ATPase of the preprotein translocase, accounts for 0.25% of the total protein in a degU32(Hy) Bacillus subtilis strain in logarithmic phase. The SecA level remained constant irrespective of the demand for exoprotein production but dropped about 12-fold during the late stationary phase. Modulation of the level of functional SecA during the exponential phase of growth affected differently the secretion of levansucrase and alpha-amylase overexpressed under the control of the sacB leader region. The level of SecA was reduced in the presence of sodium azide and in the div341 thermosensitive mutant at nonpermissive temperatures. Overproduction of SecA was obtained with a multicopy plasmid bearing secA. The gradual decrease of the SecA level reduced the yield of secreted levansucrase with a concomitant accumulation of unprocessed precursor in the cells, while an increase in the SecA level resulted in an elevation of the production of exocellular levansucrase. In contrast, alpha-amylase secretion was almost unaffected by high concentrations of sodium azide or by very low levels of SecA. Secretion defects were apparent only under conditions of strong SecA deprivation of the cell. These data demonstrate that the alpha-amylase and levansucrase precursors markedly differ in their dependency on SecA for secretion. It is suggested that these precursors differ in their binding affinities for SecA.

Activation of Bacillus licheniformis alpha-amylase through a disorder-->order transition of the substrate-binding site mediated by a calcium-sodium-calcium metal triad

BACKGROUND

The structural basis as to how metals regulate the functional state of a protein by altering or stabilizing its conformation has been characterized in relatively few cases because the metal-free form of the protein is often partially disordered and unsuitable for crystallographic analysis. This is not the case, however, for Bacillus licheniformis alpha-amylase (BLA) for which the structure of the metal-free form is available. BLA is a hyperthermostable enzyme which is widely used in biotechnology, for example in the breakdown of starch or as a component of detergents. The determination of the structure of BLA in the metal-containing form, together with comparisons to the apo enzyme, will help us to understand the way in which metal ions can regulate enzyme activity.

RESULTS

We report here the crystal structure of native, metal-containing BLA. The structure shows that the calcium-binding site which is conserved in all alpha-amylases forms part of an unprecedented linear triadic metal array, with two calcium ions flanking a central sodium ion. A region around the metal triad comprising 21 residues exhibits a conformational change involving a helix unwinding and a disorder-->order transition compared to the structure of metal-free BLA. Another calcium ion, not previously observed in alpha-amylases, is located at the interface between domains A and C.

CONCLUSIONS

We present a structural description of a major conformational rearrangement mediated by metal ions. The metal induced disorder-->order transition observed in BLA leads to the formation of the extended substrate-binding site and explains on a structural level the calcium dependency of alpha-amylases. Sequence comparisons indicate that the unique Ca-Na-Ca metal triad and the additional calcium ion located between domains A and C might be found exclusively in bacterial alpha-amylases which show increased thermostability. The information presented here may help in the rational design of mutants with enhanced performance in biotechnological applications.