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The Two Faces of Pal: Elucidating the Two Orientations of Pal Protein in Escherichia coli

Pal (Peptidoglycan associated lipoprotein) is an outer membrane lipoprotein found in many Gram‐negative bacteria, including Escherichia coli (E. coli). The exact function of Pal in E. coli is unknown; however, Pal has been well studied for its interactions in the periplasmic space, specifically in the Tol‐Pal complex. We have shown, for the first time, that Pal can also be found in a surface exposed orientation. We employed a biotinylation labeling technique to quantify the two orientations of Pal (inward and outward facing). Results from our experiments suggest that less than 25% of total Pal is surface exposed. We also test the hypothesis that the OmpA transmembrane protein aids in “flipping” Pal to the cell surface. The same biotinylation experiment was performed using an OmpA deletion mutant, demonstrating that a similar amount of Pal was surface exposed in the presence and absence of OmpA. We consider the biological implications of Pal's dual orientation.