A New Factor LapD Is Required for the Regulation of LpxC Amounts and Lipopolysaccharide Trafficking

Abstract

Lipopolysaccharide (LPS) constitutes the major component of the outer membrane and is essential for bacteria, such as Escherichia coli. Recent work has revealed the essential roles of LapB and LapC proteins in regulating LPS amounts; although, if any additional partners are involved is unknown. Examination of proteins co-purifying with LapB identified LapD as a new partner. The purification of LapD reveals that it forms a complex with several proteins involved in LPS and phospholipid biosynthesis, including FtsH-LapA/B and Fab enzymes. Loss of LapD causes a reduction in LpxC amounts and vancomycin sensitivity, which can be restored by mutations that stabilize LpxC (mutations in lapB, ftsH and lpxC genes), revealing that LapD acts upstream of LapB-FtsH in regulating LpxC amounts. Interestingly, LapD absence results in the substantial retention of LPS in the inner membranes and synthetic lethality when either the lauroyl or the myristoyl acyl transferase is absent, which can be overcome by single-amino acid suppressor mutations in LPS flippase MsbA, suggesting LPS translocation defects in ΔlapD bacteria. Several genes whose products are involved in cell envelope homeostasis, including clsA, waaC, tig and micA, become essential in LapD’s absence. Furthermore, the overproduction of acyl carrier protein AcpP or transcriptional factors DksA, SrrA can overcome certain defects of the LapD-lacking strain.

LapB (YciM) orchestrates protein–protein interactions at the interface of lipopolysaccharide and phospholipid biosynthesis

“…Since LPS and PL biosynthesis share the same precursor, FabZ would ideally be positioned in the same membrane biogenesis complex, which would facilitate the coordination of the flow of R‐3‐hydroxymyristoyl‐ACP into the appropriate direction. Since FabZ was found to co‐purify with LapB, such an interaction was suspected but never proven to be direct (Wieczorek et al, 2022). All four of our protein–protein interaction assays met the hypothesis that FabZ is a direct interaction partner of LapB.…”