The essential protein YejM functions through YciM/FtsH to control lipopolysaccharide biosynthesis in Escherichia coli

Abstract

Gram-negative bacteria have an outer membrane(OM) which functions as an effective permeability barrier responsible for their high antibiotic resistance. Understanding the mechanisms by which the OM forms and responds to environmental stress in an important strategy in identifying novel drug targets. The OM is asymmetric, with lipopolysaccharide (LPS) in its outer leaflet and glycerophospholipids (PL) in its inner leaflet. Maintenance of OM lipid asymmetry is crucial for its barrier function and is highly dependent on proper LPS biosynthesis. The LpxC deacetylase catalyzes the rate limiting step of LPS biosynthesis; its activity is tightly regulated through degradation by the FtsH protease in conjunction with its adaptor protein YciM. However, the mechanism by which LpxC degradation is coordinated to maintain proper LPS levels is not completely understood. A screen performed for suppressors of the OM defects caused by truncation of the essential protein of unknown function YejM yielded mutations in lpxC and yciM. It was shown that lpxC and yciM mutations which increase LPS levels rescue truncated yejM defects, while those that decrease LPS levels are synthetically lethal with truncated yejM. The yciM gene is demonstrated to be epistatic to yejM, as defects caused by truncated yejM are dependent on the presence of yciM. Finally, mutations that inhibit YciM activity or expression are shown to suppress depletion of YejM. This data suggests that YejM functions in modulating the proteolysis of LpxC through inhibition of YciM/FtsH, thus maintaining a level of LPS that is suitable for proper cell growth.