Crystal structure and functional characterization of OmpK36, the osmoporin of Klebsiella pneumoniae

BACKGROUND

Porins are channel-forming membrane proteins that confer solute permeability to the outer membrane of Gram-negative bacteria. In Escherichia coli, major nonspecific porins are matrix porin (OmpF) and osmoporin (OmpC), which show high sequence homology. In response to high osmolarity of the medium, OmpC is expressed at the expense of OmpF porin. Here, we study osmoporin of the pathogenic Klebsiella pneumoniae (OmpK36), which shares 87% sequence identity with E. coliOmpC in an attempt to establish why osmoporin is best suited to function at high osmotic pressure.

RESULTS

The crystal structure of OmpK36 has been determined to a resolution of 3.2 A by molecular replacement with the model of OmpF. The structure of OmpK36 closely resembles that of the search model. The homotrimeric structure is composed of three hollow 16-stranded antiparallel beta barrels, each delimiting a separate pore. Most insertions and deletions with respect to OmpF are found in the loops that protrude towards the cell exterior. A characteristic ten-residue insertion in loop 4 contributes to the subunit interface. At the pore constriction, the replacement of an alanine by a tyrosine residue does not alter the pore profile of OmpK36 in comparison with OmpF because of the different course of the mainchain. Functionally, as characterized in lipid bilayers and liposomes, OmpK36 resembles OmpC with decreased conductance and increased cation selectivity in comparison with OmpF.

CONCLUSIONS

The osmoporin structure suggests that not an altered pore size but an increase in charge density is the basis for the distinct physico-chemical properties of this porin that are relevant for its preferential expression at high osmotic strength.

Voltage sensing in the PhoE and OmpF outer membrane porins of Escherichia coli: role of charged residues

The porins PhoE and OmpF form anion and cation-selective pores, respectively, in the outer membrane of Escherichia coli. Each monomer of these trimeric proteins consists of a 16-stranded beta-barrel, which contains a constriction at half the height of the channel. The functional significance of a transverse electrical field that is formed by charged amino acid residues within the constriction zone was investigated. For this purpose, the PhoE residues R37, R75, K18 and E110 were substituted by neutral amino acids. The mutant pores allowed an increased permeation of beta-lactam antibiotics across the outer membrane in vivo, although the single channel conductance, measured in planar lipid bilayers, was not increased or even slightly decreased. Replacement of the positively charged residues resulted in a decreased voltage sensitivity, whereas the substitution of a negatively charged residue resulted in an increased voltage sensitivity. Similar substitutions in OmpF caused the opposite effects, i.e. the substitution of positive and negative charges resulted in increased and decreased voltage sensitivity, respectively. Together, the results suggest that opposite charges, i.e. positive charges in anion-selective and negative charges in cation-selective porins, act as sensors for voltage gating.