A biological porin engineered into a molecular, nanofluidic diode

We changed the nonrectifying biological porin OmpF into a nanofluidic diode. To that end, we engineered a pore that possesses two spatially separated selectivity filters of opposite charge where either cations or anions accumulate. The observed current inhibition under applied reverse bias voltage reflects, we believe, the creation of a zone depleted of charge carriers, in a sense very similar to what happens at the np junction of a semiconductor device.

Dissection and modulation of the four distinct activities of nisin by mutagenesis of rings A and B and by C-terminal truncation

Nisin A is a pentacyclic antibiotic peptide produced by various Lactococcus lactis strains. Nisin displays four different activities: (i) it autoinduces its own synthesis; (ii) it inhibits the growth of target bacteria by membrane pore formation; (iii) it inhibits bacterial growth by interfering with cell wall synthesis; and, in addition, (iv) it inhibits the outgrowth of spores. Here we investigate the structural requirements and relevance of the N-terminal thioether rings of nisin by randomization of the ring A and B positions. The data demonstrate that: (i) mutation of ring A results in variants with enhanced activity and a modulated spectrum of target cells; (ii) for the cell growth-inhibiting activity of nisin, ring A is rather promiscuous with respect to its amino acid composition, whereas the bulky amino acid residues in ring B abolish antimicrobial activity; (iii) C-terminally truncated nisin A mutants lacking rings D and E retain significant antimicrobial activity but are unable to permeabilize the target membrane; (iv) the dehydroalanine in ring A is not essential for the inhibition of the outgrowth of Bacillus cells; (v) some ring A mutants have significant antimicrobial activities but have decreased autoinducing activities; (vi) the opening of ring B eliminates antimicrobial activity while retaining autoinducing activity; and (vii) some ring A mutants escape the nisin immune system(s) and are toxic to the nisin-producing strain NZ9700. These data demonstrate that the various activities of nisin can be engineered independently and provide a basis for the design and synthesis of tailor-made analogs with desired activities.

Production of dehydroamino acid-containing peptides by Lactococcus lactis

Nisin is a pentacyclic peptide antibiotic produced by some Lactococcus lactis strains. Nisin contains dehydroresidues and thioether rings that are posttranslationally introduced by a membrane-associated enzyme complex, composed of a serine and threonine dehydratase NisB and the cyclase NisC. In addition, the transporter NisT is necessary for export of the modified peptide. We studied the potential of L. lactis expressing NisB and NisT to produce peptides whose serines and threonines are dehydrated. L. lactis containing the nisBT genes and a plasmid coding for a specific leader peptide fusion construct efficiently produced peptides with a series of non-naturally occurring multiple flanking dehydrobutyrines. We demonstrated NisB-mediated dehydration of serines and threonines in a C-terminal nisin(1-14) extension of nisin, which implies that also residues more distant from the leader peptide than those occurring in prenisin or any other lantibiotic can be modified. Furthermore, the feasibility and efficiency of generating a library of peptides containing dehydroresidues were demonstrated. In view of the particular shape and reactivity of dehydroamino acids, such a library provides a novel source for screening for peptides with desired biological and physicochemical properties.

Sec-mediated transport of posttranslationally dehydrated peptides in Lactococcus lactis

Nisin is a lanthionine-containing antimicrobial peptide produced by Lactococcus lactis. Its (methyl)lanthionines are introduced by two posttranslational enzymatic steps involving the dehydratase NisB, which dehydrates serine and threonine residues, and the cyclase NisC, which couples these dehydrated residues to cysteines, yielding thioether-bridged amino acids called lanthionines. The prenisin is subsequently exported by the ABC transporter NisT and extracellularly processed by the peptidase NisP. L. lactis expressing the nisBTC genes can modify and secrete a wide range of nonlantibiotic peptides. Here we demonstrate that in the absence of NisT and NisC, the Sec pathway of L. lactis can be exploited for the secretion of dehydrated variants of therapeutic peptides. Furthermore, posttranslational modifications by NisB and NisC still occur even when the nisin leader is preceded by a Sec signal peptide or a Tat signal peptide 27 or 44 amino acids long, respectively. However, transport of fully modified prenisin via the Sec pathway is impaired. The extent of NisB-mediated dehydration could be improved by raising the intracellular concentration NisB or by modulating the export efficiency through altering the signal sequence. These data demonstrate that besides the traditional lantibiotic transporter NisT, the Sec pathway with an established broad substrate range can be utilized for the improved export of lantibiotic enzyme-modified (poly)peptides.

Ca2+ selectivity of a chemically modified OmpF with reduced pore volume

We studied an E. coli OmpF mutant (LECE) containing both an EEEE-like locus, typical of Ca(2+) channels, and an accessible and reactive cysteine. After chemical modification with the cysteine-specific, negatively charged (-1e) reagents MTSES or glutathione, this LECE mutant was tested for Ca(2+) versus alkali metal selectivity. Selectivity was measured by conductance and zero-current potential. Conductance measurements showed that glutathione-modified LECE had reduced conductance at Ca(2+) mole fractions <10(-3). MTSES-modified LECE did not. Apparently, the LECE protein is (somehow) a better Ca(2+) chelator after modification with the larger glutathione. Zero-current potential measurements revealed a Ca(2+) versus monovalent cation selectivity that was highest in the presence of Li(+) and lowest in the presence of Cs(+). Our data clearly show that after the binding of Ca(2+) the LECE pore (even with the bulky glutathione present) is spacious enough to allow monovalent cations to pass. Theoretical computations based on density functional theory combined with Poisson-Nernst-Planck theory and a reduced pore model suggest a functional separation of ionic pathways in the pore, one that is specific for small and highly charged ions, and one that accepts preferentially large ions, such as Cs(+).

Conductance and selectivity fluctuations in D127 mutants of the bacterial porin OmpF

A recent molecular dynamics study questioned the protonation state and physiological role of aspartate 127 (D127) of E. coli porin OmpF. To address that question we isolated two OmpF mutants with D127 either neutralized (D127N) or replaced by a positively charged lysine (D127K). The charge state of the residue at position 127 has clear effects on both conductance and selectivity. The D127K but not the D127N mutant expresses resilient conductance and selectivity fluctuations. These fluctuations reflect, we think, either changes in the ionization state of K127 and/or transitions between unstable subconformations as induced by the electrostatic repulsion between two positively charged residues, K127 and the nearby R167. Our results slightly favor the view that in WT OmpF residue D127 is deprotonated. As for the role of D127 in OmpF functionality, the gating of both mutants shows very similar sensitivity toward voltage as WT OmpF. Moreover, the current fluctuations of the D127K mutant were observed also in the absence of an applied electric field. We therefore dismiss D127 as a key residue in the control mechanism of the voltage-dependent gating of OmpF.

Chemical modification of the bacterial porin OmpF: gain of selectivity by volume reduction

OmpF is an essentially nonselective porin isolated from the outer membrane of Escherichia coli. Here we report on the manipulation of the ion selectivity of OmpF by chemical modification with MTS reagents (MTSET, MTSEA, and MTSES) and the (rather bulky) tripeptide glutathione, all cysteine specific. When recorded in a gradient of 0.1//1 M CaCl2 or 0.1//1 M NaCl, pH 7.4 solutions, measured reversal potentials of the most cation-selective modified mutants were (virtually) identical to the Nernst potential of Ca2+ or Na+. Compared to this full cation selectivity, the anion-selective modified mutants performed somewhat less but nevertheless showed high anion selectivity. We conclude that a low permanent charge in combination with a narrow pore can render the same selectivity as a highly charged but wider pore. These results favor the view that both the electrostatic potential arising form the fixed charge in the pore and the space available at the selectivity filter contribute to the charge selection (i.e., cation versus anion selectivity) of a biological ion channel.

Permeation properties of an engineered bacterial OmpF porin containing the EEEE-locus of Ca2+ channels

The selectivity filter of the bacterial porin OmpF carries a small net charge close to -1 e and is therefore only slightly cation-selective. Calcium channels, on the other hand, contain four negatively charged glutamates, the EEEE-locus, and are among the most selective cation channels known. We aimed to turn the essentially nonselective OmpF into a Ca2+-selective channel. To that end, two additional glutamates (R42E and R132E) were introduced in the OmpF constriction zone that already contains D113 and E117. Mutant OmpF containing this DEEE-locus has a high Ca2+ over Cl- selectivity and a Na+ current with a strongly increased sensitivity to 1 mM Ca2+. The charge/space competition model, initially applied to the L-type Ca2+ channel, identifies the fixed charge and filter volume as key determinants of ion selectivity, with the precise atomic arrangement having only second-order effects. By implication, the reproduction of fixed charge and filter volume should transform two channels into channels of similar selectivity, even if the two belong to entirely different ion channel families, as is the case for OmpF and the L-type Ca2+ channel. The results presented here fit quite well in the framework of charge/space competition theory.

Crystallization of a genetically engineered water-soluble primary penicillin target enzyme. The high molecular mass PBP2x of Streptococcus pneumoniae

A genetically engineered water-soluble derivative of PBP2x of Streptococcus pneumoniae has been produced, purified and crystallized in a form suitable for X-ray diffraction analysis. The best crystals have been grown at 15 degrees C, from solutions containing 8% polyethylene glycol 10,000 at pH values ranging from 3.9 to 6.0. These crystals diffract to a resolution of 3.5 A and have a space group P6(1)22 (or enantiomorph) with unit cell dimensions of a = b = 162.2 A, c = 171.8 A, alpha = beta = 90 degrees, gamma = 120 degrees. The molecular mass and cell dimensions suggest that there is one molecule of enzyme per asymmetric unit. The breakdown of a chromogenic cephalosporin derivative diffused into a crystal reveals clearly that the enzyme is active in the crystalline state.