Figure 1

Topologies represented in Gram-negative bacterial inner membrane include (left to right) polytopic transmembrane proteins, monotopic transmembrane proteins and lipoproteins on the periplasmic side of the membrane which are anchored via a lipid moeity covalently attached to the N-terminal cysteine ("CD", where "D" denotes an Asp outer membrane avoidance signal at position 2 (Masuda et al. 2002)). The outer membrane also contains lipoproteins, usually on the inner leaflet exposed to the periplasm, however unlike the inner membrane the outer membrane contains ß-barrel membrane proteins ("beta"), some with large extracellular domains exposed on the surface. Akin to the Gram-negative inner membrane, the Gram-positive inner membrane contains mono and polytopic transmembrane proteins and lipoproteins. Gram-positive bacteria also display surface proteins associated covalently or non-covalently with the cell wall peptidoglycan layer via a number of "surface motifs", such as the LPxTG, LysM. Some proteins are also secreted into the extracellular milieu. A subset of Gram-positive bacteria (the Acinetobacterace) have also been shown to contain ß-barrel membrane proteins in their plasma membrane.