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 suppre ssor of htrB mutations, has been proposed to transport nascent core-li pid A molecules across the inner membrane (Polissi, k, and Georgopoulo s, C. (1996) Mol. Microbiol. 20, 1221-1233), msbA is an essential E. c oli gene with high sequence similarity to mammalian Mdr proteins and c ertain types of bacterial ABC transporters, htrB is required for growt h above 32 degrees C and encodes the lauroyltransferase that acts afte r Kdo addition during lipid A biosynthesis (Clementz, T,, Bednarski, J ., and Raetz, C. R. H. (1996) J. Biol. Chem. 271, 12095-12102). By usi ng 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, tetraacyla ted lipid A species and glycerophospholipids accumulate in the inner m embrane. Extra copies of the cloned msbA gene restore the ability of h trB mutants to grow at 42 degrees C, but they do not increase the exte nt of lipid A acylation. However, a significant fraction of the tetraa cylated lipid A species that accumulate in htrB mutants are transporte d to the outer membrane in the presence of extra copies of msbA. E. co li strains in which msbA synthesis is selectively shut off at 42 degre es C accumulate hexa-acylated lipid A and glycerophospholipids in thei r inner membranes. Our results support the view that MsbA plays a role in lipid A and possibly glycerophospholipid transport. The tetra-acyl ated lipid A precursors that accumulate in htrB mutants may not be tra nsported as efficiently by MsbA as are penta- or hexaacylated lipid A species.