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.

Ultrahigh Ionic Exclusion through Carbon Nanomembranes

“…This suggests that the CNM with a thickness of 1.2 nm can repel the penetration of ions like a lipid bilayer does but even at a reduced thickness. The measured CNM resistance is in the same order of magnitude as for lipid bilayers incorporated with a single ion channel . In previous work, atomic force microscopy (AFM) images revealed a channel density of 10 18 m −2 for TPT CNMs.…”

Regulation of Gene Expression in Shewanella oneidensis MR-1 during Electron Acceptor Limitation and Bacterial Nanowire Formation

“…Since the level of OmpW protein is increased under oxygen electron acceptor limitation (33), one hypothesis is that it may facilitate the extracellular transport of cations that might occur in concert with extracellular electron transport as a means of maintaining electrical neutrality. The recently characterized OmpW of Caulobacter crescentus has been shown to transport cations (59). This might explain the decreased current output in microbial fuel cells in the S. oneidensis mutant that lacks OmpW (34).…”

Following the terrestrial tracks of Caulobacter - redefining the ecology of a reputed aquatic oligotroph

“…The annotation of carbohydrate-active enzymes (CAZymes) was based on diamond BLASTx protein homology searches against a local CAZy database (downloaded 19 August 2015). The following glycosyl hydrolase families were deemed endoglucanases involved in cellulose degradation: 5,6,7,8,9,12,26,44,45,48,51,61,74,81, and 131 [52][53][54][55][56][57][58][59][60]. ORF prediction was performed with Prodigal ( [61]; v. 2.6.2) prior to BLAST searches.…”

“…Their ability to adhere to surfaces using holdfast protein and their dimorphic lifecycle have also been interpreted as adaptations to aquatic, oligotrophic environments [8,9]. The widespread acceptance of this ecophysiological profile is apparent in the general lack of supporting citation when referred to in the literature [6,8,[10][11][12][13][14][15].…”