Molecular Mechanisms underlying membrane localization and qualify control of lipoproteins in Escherichia coli cell surface

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An E. coli mutant, which transports the inner membrane-specific lipoprotein to the outer membrane, was isolated. The mutant carried the Asp to Pro mutation in Lo1C subunit of the Lo1CDE complex. The inner membrane-specific sorting signal was found to be a Lo1CDE avoidance signal, thereby causing the retention of lipoproteins in the inner membrane. It was then found that the Lo1CDE avoidance function was affected by phospholipids compositions.The crystal structures of Lo1A and Lo1B were solved. Their structures were very similar despite that their amino acid sequences were dissimilar. Lo1A and Lo1B have a hydrophobic cavity, which is most likely the binding site for lipoproteins. Functionally important Trp residues in Lo1B were identified. Arg at position 43 of Lo1A was mutated to other residues. Activities of these mutants revealed that differences in the strength of hydrophobic interaction with lipoproteins between Lo1A and Lo1B are critically important for efficient transfer of lipoproteins from Lo1A to Lo1B. Isolation of Lo1A mutants suggested that the hydrophobic cavity undergoes opening and closing upon the binding and release of lipoproteins. Lo1CDE was purified with tightly associated lipoproteins. This is the first example of an ABC transporter that can be isolated with its substrate.It was found that Pseudomonas aeruginosa also possesses five Lo1 proteins, which play an important role in the outer membrane sorting of lipoproteins. However, the sorting signal in P. aeruginosa was found to be different from that in E. coli.