Membrane biogenesis in Escherichia coli: effects of a secA mutation.

In Escherichia coli K-12, temperature-sensitive mutations in the secA gene have been shown to interfere with protein export. Here we show that the effect of a secA mutation is strongly pleiotropic on membrane biogenesis. Freeze-fracture experiments as well as cryosections of the cells revealed the appearance of intracytoplasmic membranes upon induction of the SecA phenotype. The permeability barrier of the outer membrane to detergents was lost. Two alterations in the outer membrane may be responsible for this effect, namely the reduced amounts of outer membrane proteins, or the reduction of the length of the core oligosaccharide of the lipopolysaccharide, which was observed in phage-sensitivity experiments and by SDS-polyacrylamide gel electrophoresis. Phospholipid analysis of the secA mutant, grown under restrictive conditions, revealed a lower content of the negatively charged phospholipid cardiolipin and of 18:1 fatty acid compared to those of the parental strain grown under identical conditions. These results are in line with the hypothesis that protein export and lipid metabolism are coupled.