A molecular safeguard in Escherichia coli Braun's lipoprotein (Lpp) biogenesis

Braun's lipoprotein (Lpp) is the most abundant protein present in Escherichia coli. Lpp is an outer membrane (OM) lipoprotein that contributes to the mechanical integrity of the cell envelope by tethering the OM to the underlying peptidoglycan (PGN). It has long been known that Lpp accumulation at the inner membrane (IM) is toxic due to aberrant tethering of the IM to the PGN. However, it is not known if there exists a cellular mechanism that mitigates such anomalous Lpp-mediated linkages. Here, we report the identification of DcrB, an inner membrane lipoprotein, as a molecular safeguard of Lpp biogenesis in E. coli. We demonstrate that DcrB promotes E. coli viability under conditions of lipoprotein maturation stress, and that in its absence Lpp-PGN linkages are altered. Genetic and biochemical tests reveal that DcrB's function is in promoting the efficiency of Lpp maturation, likely through a direct interaction. Overall, our results are consistent with two, albeit not mutually exclusive, roles for DcrB in Lpp biogenesis: promoting Lpp maturation and ameliorating the formation of deleterious IM-Lpp-PGN linkages.IMPORTANCELpp, an outer membrane (OM) lipoprotein, was discovered more than five decades ago as the first member of the bacterial lipoprotein family, which constitutes a group of important cell envelope proteins. Lpp crosslinks the OM to the underlying peptidoglycan (PGN) and contributes to the structural integrity of the cell envelope. However, Lpp-mediated crosslinks from the inner membrane (IM) to the PGN are toxic. Here, we identify an IM lipoprotein, DcrB, that binds Lpp and improves the efficiency of Lpp's journey across the IM. In so doing, DcrB attenuates the formation of aberrant IM-PGN linkages. Overall, this study reveals the existence of a critical safeguard in the biogenesis of Lpp, a paradigmatic bacterial lipoprotein.