The Outer Membrane Protein OmpW Forms an Eight-stranded β-Barrel with a Hydrophobic Channel

Abstract

Escherichia coli OmpW belongs to a family of small outer membrane proteins that are widespread in Gram-negative bacteria. Their functions are unknown, but recent data suggest that they may be involved in the protection of bacteria against various forms of environmental stress. To gain insight into the function of these proteins we have determined the crystal structure of E. coli OmpW to 2.7-Å resolution. The structure shows that OmpW forms an 8-stranded ␤-barrel with a long and narrow hydrophobic channel that contains a bound n-dodecyl-N,N-dimethylamine-N-oxide detergent molecule. Single channel conductance experiments show that OmpW functions as an ion channel in planar lipid bilayers. The channel activity can be blocked by the addition of n-dodecyl-N,Ndimethylamine-N-oxide. Taken together, the data suggest that members of the OmpW family could be involved in the transport of small hydrophobic molecules across the bacterial outer membrane. The outer membrane (OM)2 of Gram-negative bacteria is a protective barrier that hinders the permeability of both hydrophilic and hydrophobic compounds, because of the presence of lipopolysaccharide (LPS) within the outer leaflet of the OM (1). To obtain nutrients and other molecules that are necessary for growth and function of the cell, Gramnegative bacteria have channels within their OM that facilitate uptake of these molecules. With respect to the transport of small, hydrophilic substances, these channels can be divided in three classes, based on their mode of transport (1): general porins, substrate-specific transporters, and active transporters. A wealth of structural and functional information is available for many of these OM channel proteins, which form monomeric or trimeric barrels that are each composed of 12-22 antiparallel ␤-strands. In addition to OM proteins with established transport functions, the OM also contains a considerable number of smaller, monomeric ␤-barrels that are composed of 8 or 10 ␤-strands. These proteins have been implicated in a wide range of functions including OM lipid metabolism, cell adhesion, and structural functions. One of these small OM proteins is OmpA from Escherichia coli, which belongs to a protein family with a number of established and putative functions, the most important of which is to provide structural stability to the cell via interactions of its C-terminal domain with the periplasmic peptidoglycan layer (1). Another member of the small OM protein family is NspA from Neisseria meningitidis, which belongs to the Opa family of proteins that are thought to mediate adhesion to host cells (2).A fundamental question is whether these small barrels can function as transport channels. Arguing against this possibility are the crystal structures that have been determined for several of these proteins, and which do not show continuous channels that would be consistent with transport functions. On the other hand, it has been shown that, at least in vitro, OmpA forms both small and large ion channels (3) and is permeable to larger uncha...

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).…”

Immune response in Lutjanus erythropterus induced by the major outer membrane protein (OmpU) of Vibrio alginolyticus

“…During colonization, surface materials of the organisms, such as lipopolysaccharide and OMPs, play an important role in the adhesion to host cells (Cheng et al 2005, Hong et al 2006. OMPs of Vibrio and their role have been mainly studied in V. cholerae (Sperandio et al 1995), V. anguillarum (Waldbeser et al 1993), V. vulnificus (Litwin & Byrne 1998) and V. fischeri (Aeckersberg et al 2001), but there are very few reports on the immunological characterization of the major OMP from V. alginolyticus.…”

Effects of high-pressure carbon dioxide on proteins and DNA in Escherichia coli

“…OmpA-deficient E. coli exhibits reduced growth (Jeannin et al, 2002). OmpW is involved in protection of bacteria against various forms of environmental stress and in the transport of small hydrophobic molecules across the bacterial outer membrane (Hong et al, 2006). Lipoprotein mutant and OmpA of E. coli were downregulated, while OmpW was upregulated following HPCD treatment.…”