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.
Dutzler
R
R
Department of Structural Biology, Biozentrum University of Basel, CH-4056 Basel, Switzerland.
Rummel
G
G
Albertí
S
S
Hernández-Allés
S
S
Phale
P
P
Rosenbusch
J
J
Benedí
V
V
Schirmer
T
T
D001426:Bacterial Proteins; D001483:Base Sequence; D001692:Biological Transport; D050260:Carbohydrate Metabolism; D002463:Cell Membrane Permeability; D018360:Crystallography, X-Ray; D003902:Detergents / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D007711:Klebsiella pneumoniae / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009997:Osmotic Pressure; D018272:Porins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011487:Protein Conformation; D016415:Sequence Alignment; D012689:Sequence Homology, Nucleic Acid
D001426:Bacterial Proteins; D003902:Detergents; C113724:OmpC protein; C109237:OmpF protein; C092233:OmpK36 protein, Klebsiella pneumoniae; D018272:Porins