Redefining the essential trafficking pathway for outer membrane lipoproteins

Abstract

The outer membrane (OM) of Gram-negative bacteria is a permeability barrier and an intrinsic antibiotic resistance factor. Lipoproteins are OM components that function in cell wall synthesis, diverse secretion systems, and antibiotic efflux pumps. Moreover, each of the essential OM machines that assemble the barrier requires one or more lipoproteins. This dependence is thought to explain the essentiality of the periplasmic chaperone LolA and its OM receptor LolB that traffic lipoproteins to the OM. However, we show that in strains lacking substrates that are toxic when mislocalized, both LolA and LolB can be completely bypassed by activating an envelope stress response without compromising trafficking of essential lipoproteins. We identify the Cpx stress response as a monitor of lipoprotein trafficking tasked with protecting the cell from mislocalized lipoproteins. Moreover, our findings reveal that an alternate trafficking pathway exists that can, under certain conditions, bypass the functions of LolA and LolB, implying that these proteins do not perform any truly essential mechanistic steps in lipoprotein trafficking. Instead, these proteins' key function is to prevent lethal accumulation of mislocalized lipoproteins.

Lipoprotein Transport: Greasing the Machines of Outer Membrane Biogenesis

“…The apparent lack of LolB in some species prompted a recent re‐evaluation of the essentiality of this protein in E. coli . Depletion of LolB was found to cause two major toxicities that impaired viability.…”

“…Depletion of LolB was found to cause two major toxicities that impaired viability. The first was caused by mislocalization of Lpp to the IM and could be alleviated by deleting the gene or removing the C‐terminal Lys required for cell wall attachment . The second toxicity resulted from hyper‐activation of the Rcs stress response and was likely caused by IM mislocalization of the RcsF sensory lipoprotein .…”

“…The first was caused by mislocalization of Lpp to the IM and could be alleviated by deleting the gene or removing the C‐terminal Lys required for cell wall attachment . The second toxicity resulted from hyper‐activation of the Rcs stress response and was likely caused by IM mislocalization of the RcsF sensory lipoprotein . Such RcsF mislocalization was known to inappropriately activate the system even in the absence of stress .…”

“…Remarkably, Rcs toxicity is due to upregulation of just a single regulon member, a small OM lipoprotein OsmB. Cells lacking both lpp and osmB grew well when LolB‐levels were depleted . So, highly efficient lipoproteins transport via LolB is demanded by the production of Lpp and OsmB.…”

“…Understanding this diversity is an important goal in the context of ongoing efforts to identify new classes of antibiotics that inhibit OM biogenesis. Moreover, it was recently discovered that a series of suppressor mutations can entirely bypass the essentiality of both LolA and LolB in E. coli , revealing that an alternate transport mechanism exists . The goal of this work is to reconsider lipoprotein trafficking from a broader proteobacterial perspective.…”