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

Background: Porins are channel-forming membrane proteins that confer solute permeability to the outer membrane of Gram-negative bacteria. In Escherich ia coli, major nonspecific porins are matrix porin (OmpF) and osmoporin (Om pC), which show high sequence homology. In response to high osmolarity of t he medium, OmpC is expressed at the expense of OmpF porin. Were, we study o smoporin of the pathogenic Klebsiella pneumoniae (OmpK36), which shares 87% sequence identity with E. coli OmpC in an attempt to establish why osmopor in is best suited to function at high osmotic pressure. Results: The crystal structure of OmpK36 has been determined to a resolutio n of 3.2 A by molecular replacement with the model of OmpF. The structure o f OmpK36 closely resembles that of the search model. The homotrimeric struc ture is composed of three hollow 16-stranded antiparallel beta barrels, eac h delimiting a separate pore. Most insertions and deletions with respect to OmpF are found in the loops that protrude towards the cell exterior. A cha racteristic ten-residue insertion in loop 4 contributes to the subunit inte rface. At the pore constriction, the replacement of an alanine by a tyrosin e residue does not alter the pore profile of OmpK36 in comparison with OmpF because of the different course of the mainchain. Functionally, as charact erized in lipid bilayers and liposomes, OmpK36 resembles OmpC with decrease d 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-ch emical properties of this porin that are relevant for its preferential expr ession at high osmotic strength.