title: OmpW of Caulobacter crescentus Functions as an Outer Membrane Channel for Cations.
abstract: Caulobacter crescentus is an oligotrophic bacterium that lives in dilute organic environments such as soil and freshwater. This bacterium represents an interesting model for cellular differentiation and regulation because daughter cells after division have different forms: one is motile while the other is non-motile and can adhere to surfaces. Interestingly, the known genome of C. crescentus does not contain genes predicted to code for outer membrane porins of the OmpF/C general diffusion type present in enteric bacteria or those coding for specific porins selective for classes of substrates. Instead, genes coding for 67 TonB-dependent outer membrane receptors have been identified, suggesting that active transport of specific nutrients may be the norm. Here, we report that high channel-forming activity was observed with crude outer membrane extracts of C. crescentus in lipid bilayer experiments, indicating that the outer membrane of C. crescentus contained an ion-permeable channel with a single-channel conductance of about 120 pS in 1M KCl. The channel-forming protein with an apparent molecular mass of about 20 kDa was purified to homogeneity. Partial protein sequencing of the protein indicated it was a member of the OmpW family of outer membrane proteins from Gram-negative bacteria. This channel was not observed in reconstitution experiments with crude outer membrane extracts of an OmpW deficient C. crescentus mutant. Biophysical analysis of the C. crescentus OmpW suggested that it has features that are special for general diffusion porins of Gram-negative outer membranes because it was not a wide aqueous channel. Furthermore, OmpW of C. crescentus seems to be different to known OmpW porins and has a preference for ions, in particular cations. A putative model for OmpW of C. crescentus was built on the basis of the known 3D-structures of OmpW of Escherichia coli and OprG of Pseudomonas aeruginosa using homology modeling. A comparison of the two known structures with the model of OmpW of C. crescentus suggested that it has a more hydrophilic interior and possibly a larger diameter.
authors.affiliation: Department of Life Sciences and Chemistry, Jacobs University, Campus ring 1, 28759, Bremen, Germany.|Rudolf-Virchow-Center, DFG-Research Center for Experimental Biomedicine, University of Würzburg, 97078, Würzburg, Germany.
authors.forename: Roland
authors.initials: R
authors.lastname: Benz
authors.affiliation: Department of Microbiology and Immunology, Life Sciences Centre, 2509-2350 Health Sciences Mall, University of British Columbia, Vancouver, British Columbia, V6T 1Z3, Canada.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Jones
authors.affiliation: Department of Life Sciences and Chemistry, Jacobs University, Campus ring 1, 28759, Bremen, Germany.
authors.forename: Farhan
authors.initials: F
authors.lastname: Younas
authors.affiliation: Rudolf-Virchow-Center, DFG-Research Center for Experimental Biomedicine, University of Würzburg, 97078, Würzburg, Germany.
authors.forename: Elke
authors.initials: E
authors.lastname: Maier
authors.affiliation: Department of Physics and Earth Sciences, Jacobs University, Campus ring 1, 28759, Bremen, Germany.
authors.forename: Niraj
authors.initials: N
authors.lastname: Modi
authors.affiliation: Max-Planck Institute of Biochemistry, Department for Protein Analytics, Am Klopferspitz 18A, 82152, Martinsried, Germany.
authors.forename: Reinhard
authors.initials: R
authors.lastname: Mentele
authors.affiliation: Max-Planck Institute of Biochemistry, Department for Protein Analytics, Am Klopferspitz 18A, 82152, Martinsried, Germany.
authors.forename: Friedrich
authors.initials: F
authors.lastname: Lottspeich
authors.affiliation: Department of Physics and Earth Sciences, Jacobs University, Campus ring 1, 28759, Bremen, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Kleinekathöfer
authors.affiliation: Department of Microbiology and Immunology, Life Sciences Centre, 2509-2350 Health Sciences Mall, University of British Columbia, Vancouver, British Columbia, V6T 1Z3, Canada.
authors.forename: John
authors.initials: J
authors.lastname: Smit
mesh_terms: D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002412:Cations / Q000378:metabolism*; D016935:Caulobacter crescentus / Q000235:genetics / Q000378:metabolism*; D055785:Gene Knockdown Techniques; D007473:Ion Channels / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009154:Mutation
chemical_list: D001425:Bacterial Outer Membrane Proteins; D002412:Cations; D007473:Ion Channels