Modulations in lipid A and phospholipid biosynthesis pathways influence outer membrane protein assembly in Escherichia coli K‐12

Abstract

SummaryThe assembly defect of a mutant outer membrane protein, OmpF315, can be corrected by suppressor mutations that lower lipopolysaccharide (LPS) levels and indirectly elevate phospholipid levels. One such assembly suppressor mutation, asmB1, is an allele of lpxC (envA) whose product catalyses the first ratelimiting step in the lipid A (LPS) biosynthesis pathway. Besides reducing LPS levels, asmB1 confers sensitivity to MacConkey medium. A mutation, sabA1, that reverses the MacConkey sensitivity phenotype of asmB1 maps within fabZ (whose product is needed for phospholipid synthesis from a precursor) is also required for lipid A synthesis. In addition to reversing MacConkey sensitivity, the sabA1 mutation reverses the OmpF315 assembly suppression phenotype of asmB1. These results show that OmpF315 assembly suppression by asmB1, which is achieved by lowering LPS levels, can be averted by a subsequent aberration in phospholipid synthesis at a point where the biosynthetic pathways for these two lipid molecules split. OmpF315 assembly suppression can also be achieved in an asmB þ background where FabZ expression is increased. The data obtained in this study provide genetic evidence that elevated phospholipid levels and/or phospholipid to LPS ratios are necessary for assembly suppression.

Compromised Outer Membrane Integrity in Vibrio cholerae Type II Secretion Mutants

“…Besides providing a means for colicins and filamentous phages to enter bacterial cells, the Tol-Pal system also has been suggested to maintain the integrity/stability of the outer membrane (24) and to participate in cell division by assisting in the invagination of the outer membrane during septum formation (23). Other cell envelope mutants with phenotypes resembling the eps phenotype arise from mutations in genes encoding one of the most abundant outer membrane proteins, Lpp (86,91), the Tat system (50, 84), or in genes coding for proteins involved in the biosynthesis of phospholipids and LPS (42).…”

The membrane phospholipids of Neisseria meningitidis and Neisseria gonorrhoeae as characterized by fast atom bombardment mass spectrometry

“…The role of phospholipids in maintaining the integrity of the outer membrane has been demonstrated. Perturbation of outer-membrane biogenesis in E. coli by mutations which decrease lipid A synthesis (Karow et al, 1992) and suppress outer-membrane protein assembly (Kloser et al, 1998) results in elevated levels of phospholipid synthesis and incorporation into the outer membrane. Since both LPS and phospholipid pathways share common fatty acid precursors, this effect may be mediated by the shift in the flow of these precursors from one pathway to the other.…”

Intragenic Suppressors of an OmpF Assembly Mutant and Assessment of the Roles of Various OmpF Residues in Assembly through Informational Suppressors

“…By utilizing a severely defective assembly mutant, OmpF315, suppressor mutations conferring a Dex ϩ phenotype were isolated. Further examination revealed that all the suppressor mutations mapped outside the ompF gene (11,12,15,17). A factor contributing to our inability to isolate intragenic suppressor mutations could have been the availability of a large number of potential chromosomal targets other than ompF that can be mutated to yield a Dex ϩ phenotype.…”

“…Naturally, compositional changes in any of these three compartments can potentially influence the biogenesis of OMPs. Accordingly, inner membrane components influence protein translocation (for a review, see reference 21), periplasmic components influence the early stages of OMP assembly (2,14,19,23), and outer membrane components such as lipopolysaccharide (LPS) can affect the later stages of OMP assembly (6,8,11,12,13,22,24).…”

“…Reversion analysis of these temperature-sensitive assembly mutants resulted in extragenic suppressor mutations in asm genes, whose products influence the outer membrane lipid environment (6,11,12,17). In this study, we focused our attention on intragenic suppressor mutations in hopes of identifying additional residues within OmpF that play a role during assembly.…”