Functional analyzes on an ABC transporter involved in the selective release of outer membrane lipoproteins from membranes

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We identified a unique ABC transporter, LolCDE, which is responsible for the localization of lipoproteins from the inner to the outer membrane. The ATPase activity of LolCDE was stimulated in an outer membrane lipoprotein-dependent manner, indicating that LolCDE directly recognizes the sorting signal of lipoproteins. The same conclusion was obtained from a genetic analysis on a mutant in which lipoproteins possessing inner membrane sorting signals are localized to the outer membrane. We proposed that the inner membrane sorting signal acts as an avoidance signal on the basis of that inner membrane lipoproteins are not recognized by LolCDE. We showed that N-acylation of the cysteine residue is essential for the Lol system-dependent localization of lipoproteins to the outer membrane. Biochemical analyzes of a mutant LolA protein revealed that LolA directly interacts with LolCDE to transfer lipoproteins. The LolCDE gene as well as lolA and lolB was found to be essential for cell growth of E.coli, and almost all of Gram-negative bacteria have their homologs, suggesting that the membrane localization of lipoproteins by the Lol system is a common feature in Gram-negative bacteria.