These two proteins form noncovalent associations with the peptide components of the peptidoglycan sacculus, linking it to the outer membrane (Parsons et al., 2006), and Pal also is part of the Tol–Pal system that forms an envelope spanning complex (reviewed recently by Godlewska

et al., 2009). Interestingly, a chimera of MotB containing a variant of the peptidoglycan-binding motif from Pal instead Ion Channel Ligand Library of its native motif was able to facilitate flagellar motility (Hizukuri et al., 2009), demonstrating that the peptidoglycan interactions, rather than the specific peptidoglycan-binding motif, were critical for function. Crystal structures of MotB and its homologue MotY revealed that the peptidoglycan-binding site is wider than its counterparts in OmpA or Pal (Kojima et al., 2008; Roujeinikova, 2008). The larger binding site was suggested to mediate low affinity binding to peptidoglycan, explaining the transient nature of MotB–peptidoglycan interactions. Peptidoglycan can also

act as an anchor to counter forces such as those experienced during pilus-mediated twitching motility. The forces generated by retraction of a single T4P can reach 140 pN (Maier et al., 2002), representing one of the strongest molecular motors identified to date. Unlike that observed with flagellar motility, the peptidoglycan–pilus assembly complex association is unlikely to be Montelukast Sodium transient in nature. To prevent detachment of pili during generation of retraction forces, see more the basal complex of the pilus would need to be affixed to the peptidoglycan layer. For similar reasons, the structural support provided by the peptidoglycan layer could presumably assist in puncturing of target cells by the T3S, T4S, and T6S system apparati, processes that would exert inwardly directed forces on the bacterial cell envelope. FimV, a protein containing an LysM

peptidoglycan-binding motif, has been implicated in interactions of the T4P system with the peptidoglycan layer in P. aeruginosa (Semmler et al., 2000). The LysM motif is a ubiquitous domain that is involved in binding to peptidoglycan and chitin, presumably through direct interactions with the GlcNAc moiety shared by these two polysaccharides (Buist et al., 2008). FimV is required for twitching motility, as well as multimerization of the >1 MDa outer membrane secretin, PilQ. Mutants expressing a form of FimV lacking the LysM domain retain only ∼30% of wild-type twitching and have reduced levels of surface piliation and multimeric PilQ (Wehbi et al., 2011). As interactions of FimV with the PilMNOP inner membrane assembly complex were inferred from protein stability experiments, FimV may be involved in anchoring of the T4P apparatus within the peptidoglycan layer (Wehbi et al., 2011).