It has been shown in E. coli that deleting any of the POTRA domains other than P1 results in disruption of accessory lipoprotein interactions [57]. Similar to the E. coli BAM accessory lipoproteins, it is likely that BB0324 and BB0028 also associate with BamA through POTRA domain contacts. Future co-immunoprecipitation experiments with different B. burgdorferi BamA POTRA domain mutants as well as BB0324, and/or BB0028 mutants will help clarify exactly which NCT-501 POTRA domains are needed for BB0324 and BB0028 accessory protein binding. BB0324 is a putative BamD ortholog with a

truncated C-terminus BlastP searches and sequence analyses indicate that the BB0324 protein is a putative B. burgdorferi BamD ortholog. BamD is predicted to be ubiquitous

in diderm bacteria [10, 15, 21], and it appears to be both essential for cell survival and central to the function of the AR-13324 nmr BAM complex, as demonstrated in E. coli and in N. meningitidis [18, 21, 25, 30, 58]. It is predicted that all BamD orthologs possess N-terminal TPR domains [15], and in E. coli and N. meningitidis, BamD appears to contain two (see Figure 2). Although such structural features are still predicted for E. coli and N. meningitidis, a recently-determined crystal structure from the Rhodothermus marinus BamD confirms the presence of TPR domains within this protein [59]. Although TPRs form a characteristic helix-loop-helix structure, their propensity for sequence variation is likely a reason that we were initially unable to identify a BamD ortholog in B. burgdorferi, even though BB0324 contains tuclazepam consensus TPR sequences [27–29]. In addition, BB0324 is considerably smaller than the BamD proteins currently identified in other bacteria. The putative borrelial BamD lipoprotein has a predicted MW of ~14 kDa, which is less than half the size of proteobacterial BamD proteins from E. coli, N. meningitidis, and C. crescentus. Interestingly,

it has been proposed that the TPR domain region fulfills the major functional requirements for BamD (i.e., XAV-939 supplier binding OMPs and/or interacting with BAM components), and that the TPRs may be the only essential feature of the BamD proteins [10, 30]. This idea has been discussed in previous reports, and it originates from the discovery of a viable transposon mutant of the Neisseria gonorrhoeae BamD protein, also known as ComL [58]. As noted by Volokhina et al., this truncated mutant contains only 96 amino acids of the mature 267-residue protein, indicating that the ComL N-terminus, which comprises the TPR motifs, is sufficient for viability [30, 58]. Although viable, the ComL mutant displayed reduced colony size and was deficient in transformation competency [58]. Similarly, an E.