Surface protein machineries in Gram-negative bacteria.

The major feature that distinguishes Gram-negative bacteria from Gram-positive bacteria is the presence of both an inner and outer membrane. Both membranes contain lipoproteins, peripheral membrane proteins, α-helical membrane proteins and a host of β-barrel membrane proteins. However, the vast majority of outer membrane proteins (OMPs) are β-barrel proteins, with a few select α-helical exceptions. In this Review, we provide a general overview of many of the proteins and protein machineries essential for cell survival that are commonly found on the surface of Gram-negative bacteria. Many of these proteins also serve important roles in mediating virulence in pathogenic strains, making them promising drug and vaccine targets. Examples include the β-barrel assembly machinery (BAM) complex, which is involved in the biogenesis and assembly of OMPs; surface lipoprotein assembly modulator 1 (Slam1), which is involved in the presentation of lipoproteins at the surface; efflux pumps, which remove potentially harmful substrates; as well as the protein assemblies that comprise pili and flagella. The structures of these protein machineries and the underlying mechanisms responsible for their function create a complex and dynamic surface landscape that must adapt quickly to ever-changing environmental conditions.