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.
Highlights
The Gram-negative bacterium Pseudomonas aeruginosa is an opportunistic human pathogen associated with lung infections in cystic fibrosis patients and nosocomial infections [1]
Due to the impermeability of the OM, gramnegative bacteria have evolved three major classes of outer membrane proteins to facilitate the transport of nutrients into the cell: active transporters, general porins, and diffusion-driven specific transporters [2]
A distinctive feature of OprG is that the lumen of the barrel on the extracellular side of the OM is lined almost exclusively with hydrophobic residues (Fig. 2a): of the,45 residues with sidechains pointing towards the lumen of the barrel, only,5 are polar (Gln35, His72, Gln92 Asn120, Arg133 and Gln136 in OprG)
Summary
The Gram-negative bacterium Pseudomonas aeruginosa is an opportunistic human pathogen associated with lung infections in cystic fibrosis patients and nosocomial infections [1]. Conclusions/Significance: The 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. Perhaps the best clue for a putative function for OmpW family members is provided by sequence similarity to OM proteins present in operons dedicated to the biodegradation of small, hydrophobic molecules such as medium-chain alkanes (AlkL) and naphthalene (NahQ and DoxH) [14,15,16].
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