| Summary: | <p>Abstract</p> <p>Background</p> <p>Similar to Gram-negative bacteria, the outer membrane (OM) of the pathogenic spirochete, <it>Borrelia burgdorferi</it>, contains integral OM-spanning proteins (OMPs), as well as membrane-anchored lipoproteins. Although the mechanism of OMP biogenesis is still not well-understood, recent studies have indicated that a heterooligomeric OM protein complex, known as BAM (β-barrel assembly machine) is required for proper assembly of OMPs into the bacterial OM. We previously identified and characterized the essential β-barrel OMP component of this complex in <it>B. burgdorferi</it>, which we determined to be a functional BamA ortholog.</p> <p>Results</p> <p>In the current study, we report on the identification of two additional protein components of the <it>B. burgdorferi </it>BAM complex, which were identified as putative lipoproteins encoded by ORFs BB0324 and BB0028. Biochemical assays with a BamA-depleted <it>B. burgdorferi </it>strain indicate that BB0324 and BB0028 do not readily interact with the BAM complex without the presence of BamA, suggesting that the individual <it>B. burgdorferi </it>BAM components may associate only when forming a functional BAM complex. Cellular localization assays indicate that BB0324 and BB0028 are OM-associated subsurface lipoproteins, and <it>in silico </it>analyses indicate that BB0324 is a putative BamD ortholog.</p> <p>Conclusions</p> <p>The combined data suggest that the BAM complex of <it>B. burgdorferi </it>contains unique protein constituents which differ from those found in other proteobacterial BAM complexes. The novel findings now allow for the <it>B. burgdorferi </it>BAM complex to be further studied as a model system to better our understanding of spirochetal OM biogenesis in general.</p>
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