The Crystal Structure of OprG from Pseudomonas aeruginosa, a Potential Channel for Transport of Hydrophobic Molecules across the Outer Membrane

Abstract

BackgroundThe outer membrane (OM) of Gram-negative bacteria provides a barrier to the passage of hydrophobic and hydrophilic compounds into the cell. The OM has embedded proteins that serve important functions in signal transduction and in the transport of molecules into the periplasm. The OmpW family of OM proteins, of which P. aeruginosa OprG is a member, is widespread in Gram-negative bacteria. The biological functions of OprG and other OmpW family members are still unclear.Methodology/Principal FindingsIn order to obtain more information about possible functions of OmpW family members we have solved the X-ray crystal structure of P. aeruginosa OprG at 2.4 Å resolution. OprG forms an eight-stranded β-barrel with a hydrophobic channel that leads from the extracellular surface to a lateral opening in the barrel wall. The OprG barrel is closed off from the periplasm by interacting polar and charged residues on opposite sides of the barrel wall.Conclusions/SignificanceThe crystal structure, together with recent biochemical data, suggests that OprG and other OmpW family members form channels that mediate the diffusion of small hydrophobic molecules across the OM by a lateral diffusion mechanism similar to that of E. coli FadL.

Transcription of TP0126, Treponema pallidum Putative OmpW Homolog, Is Regulated by the Length of a Homopolymeric Guanosine Repeat

“…This peculiarity would allow the porin substrate to avoid the hydrophilic periplasmic space. Similar findings were later reported for the P. aeruginosa OprG by Touw et al (26). Whether TP0126 might be the transporter that mediates the acquisition of fatty acids (one of the many macromolecules that the syphilis agent is unable to synthesize but must scavenge from the host) in T. pallidum has yet to be shown, but it certainly appears to be an intriguing working hypothesis.…”

“…Evidence that phase variation is often (although not exclusively) reported to affect expression of surface antigens in bacterial pathogens (14,19) prompted us to begin investigating whether TP0126 could be a newly identified putative surface antigen. We provide in silico evidence that identifies the shorter TP0126 ORF to be a putative homolog of OmpW, an outer membrane porin likely involved in transporting hydrophobic molecules into the outer membrane (25,26). Circular dichroism (CD) analysis of recombinant TP0126 showed a ␤-sheet component compatible with structural homology between TP0126 and OmpW.…”

Outer Membrane Protein AlkL Boosts Biocatalytic Oxyfunctionalization of Hydrophobic Substrates in Escherichia coli

“…OmpW and OprG structures involve an eight-stranded ␤-barrel forming a hydrophobic channel. As a mechanism for hydrophobic substrate uptake, this channel was proposed to enable the transfer of hydrophobic molecules through the LPS layer, followed by their release into the outer membrane through a lateral opening (27,63,69). A similar mechanism has been described for the uptake of longchain fatty acids by FadL (25,68).…”

“…AlkL shows homology to the outer membrane proteins OmpW (29%) from E. coli and OprG (23%) from Pseudomonas aeruginosa (see the supplemental material), which might be involved in the uptake of hydrophobic molecules (27,63). OmpW and OprG structures involve an eight-stranded ␤-barrel forming a hydrophobic channel.…”

The complex that inserts lipopolysaccharide into the bacterial outer membrane forms a two-protein plug-and-barrel

“…Such movement of hydrophobic substrates between the OM bilayer and the lumen of a β-barrel has been described in several systems (22), including the long-chain fatty acid transporter FadL (23), the small porins OmpW and OprG (24)(25)(26), and the acyltransferase PagP (27). In fact, LPS is a substrate of PagP and is believed to access the active site via a lateral opening in its β-barrel wall (27).…”