Pal is a lipoprotein, with a previous analysis by Edman sequencing showing the protein to be processed at a conserved cysteine that is modified with a lipid group 
. This lipoprotein has biomedical importance, with Pal acting as an agonist for a specific isoform of Toll-like receptor (TLR2), thereby activating inflammation through cytokine release from macrophages 
. Decreased expression levels of the Pal protein has been reported to impact on bacterial virulence 
, with similar findings reported for loss-of-function mutants lacking yfgL
and smpA 
: these genes encode the BamB and BamE lipoprotein subunits of the BAM complex. Decreased expression of Pal leads to outer membrane morphology defects providing a molecular explanation to the decreased virulence of Pal-depleted bacteria.
Pal has an OmpA-type peptidoglycan binding domain 
. These domains are found in many outer and inner membrane proteins and serve to bind together structures within the bacterial cell wall. For example, the OmpA-type domain of MotB is essential to anchor the flagellum within the wall to ensure that flagellar motion translates into motility 
. Chimeric proteins have been used to show that the Pal OmpA-type domain can replace the equivalent domain in MotA to enable function of the flagellum 
. Thus, Pal serves as an anchor, and anchorage will be bestowed to whichever protein complex Pal selectively binds. After solubilisation of outer membranes with detergent, immunoprecipitation analysis proved Pal and three other lipoproteins (BamB, BamD and BamE) are bound to BamA. When the same solubilisation conditions are used and analyzed by blue-native PAGE, these four lipoproteins: Pal, BamB, BamD and BamE are associated with BamA in the 300 kDa BAM holo-complex. More stringent conditions cause Pal to be released from the core BAM complex. Thus, Pal should be considered a module of the BAM complex.
Cryo-electron microscopy of E. coli
and Pseudomonas aeruginosa
suggest that the distance between the outer and inner membranes can vary considerably and is closest in regions where no periplasmic “space” exists, where the two membranes are closely apposed to the peptidoglycan layer 
. The model proposed from these studies suggests that proteins anchoring membrane to peptidoglycan stabilize regions of relatively close proximity between the outer and inner membranes. While speculative at this stage, measurements of the POTRA domains in BamA 
have led to the suggestion that the POTRA domain might span the distance to the cytoplasmic (inner) membrane, to assist in capturing substrates during translocation into the periplasm 
. The Pal module of the BAM complex could assist in minimizing the distance the POTRA domains are required to span from the outer membrane to the sites of protein substrate entry into the periplasm.
While this manuscript was in preparation, Volokhina and co-workers reported that RmpM associates with the BAM complex in Neisseria meningitidis 
. A study from Sauri et al investigating the transport pathway of the beta-barrel protein Hbp in E. coli
found a translocation intermediate cross-linked to BamA and BamB, the periplasmic chaperone SurA and OmpA 
. Further work is needed to fully understand the subunit composition of BAM holo-complexes, but the evidence so far suggests a generalized role for OmpA-domain proteins in anchoring the BAM complex to the peptidoglycan layer of diverse Gram negative bacteria.
We do not yet understand the subunit architecture of the BAM complex, but a clue to the docking mechanism for Pal comes from the interaction of Pal and TolB in E. coli
. In E. coli
, Pal binds to a β-strand from the propeller structure of the protein TolB, through an “induced fit”: a short segment in helix 3 of Pal is unwound and reforms as a β-structure paired with the TolB β-propeller 
. The interaction of OmpA domain proteins like Pal and the BAM complex might be mediated via a similar induced fit mechanism through the BamB subunit, since BamB is predicted to be a β-propeller 
, which would make BamB a crucial bridging element in the docking of BamA to peptidoglycan.
In order to receive substrate proteins in an “assembly competent” state, the mitochondrial SAM complex is positioned closely to the protein transporter, the TOM complex 
. By minimizing the periplasmic distances involved, Pal might likewise help position the BAM complex in the proximity of the bacterial SecYEG protein transporter, to improve the efficiency of substrate protein transfer from the inner to outer membrane.