Mechanism of assembly of the outer membrane of Salmonella typhimurium. Translocation and integration of an incomplete mutant lipid A into the outer membrane.

An incomplete precursor of lipid A produced by a mutant of Salmonella typhimurium conditionally defective in synthesis of 3-deoxy-D-mannooctulosonate (KDO) (Rick, P.D., and Osborn, M.J. (1977) J. Biol. Chem. 252, 4895-4903) is poorly translocated to the outer membranes. Approximately 40% of the lipid A precursor synthesized under nonpermissive conditions was recovered in the isolated inner (cytoplasmic) membrane fraction, and the rate of translocation to outer membrane in pulse-chase experiments was only 20% that of lipopolysaccharide. However, integration of the incomplete molecule into the outer membrane appeared to be similar to that of lipopolysaccharide in stability and irreversibility. The ultimate extent of translocation pulse-labeled precursor was comparable to that of lipopolysaccharide and the process was functionally unidirectional. Little or no reverse translocation from outer to inner membrane was detected during conversion of preformed lipid A precursor to lipopolysaccharide following return to permissive conditions.