Function of Escherichia coli MsbA, an essential ABC family transporter, in lipid A and phospholipid biosynthesis

The Escherichia coli msbA gene, first identified as a multicopy suppressor of htrB mutations, has been proposed to transport nascent core-lipid A molecules across the inner membrane (Polissi, k, and Georgopoulos, C. (1996) Mol. Microbiol. 20, 1221-1233), msbA is an essential E. coli gene with high sequence similarity to mammalian Mdr proteins and certain types of bacterial ABC transporters, htrB is required for growth above 32 degrees C and encodes the lauroyltransferase that acts after Kdo addition during lipid A biosynthesis (Clementz, T,, Bednarski, J., and Raetz, C. R. H. (1996) J. Biol. Chem. 271, 12095-12102). By using a quantitative new P-32(i) labeling technique, we demonstrate that hexa-acylated species of lipid A predominate in the outer membranes of wild type E. coli labeled for several generations at 42 degrees C. In contrast, in htrB mutants shifted to 42 degrees C for 3 h, tetraacylated lipid A species and glycerophospholipids accumulate in the inner membrane. Extra copies of the cloned msbA gene restore the ability of htrB mutants to grow at 42 degrees C, but they do not increase the extent of lipid A acylation. However, a significant fraction of the tetraacylated lipid A species that accumulate in htrB mutants are transported to the outer membrane in the presence of extra copies of msbA. E. coli strains in which msbA synthesis is selectively shut off at 42 degrees C accumulate hexa-acylated lipid A and glycerophospholipids in their inner membranes. Our results support the view that MsbA plays a role in lipid A and possibly glycerophospholipid transport. The tetra-acylated lipid A precursors that accumulate in htrB mutants may not be transported as efficiently by MsbA as are penta- or hexaacylated lipid A species.