Novel mutations of the LolCDE complex causing outer membrane localization of lipoproteins despite their inner membrane-retention signals

In Gram-negative bacteria, lipoproteins are targeted to either the inner or outer membrane depending on their sorting signals. An ABC transporter LolCDE complex in Escherichia coli releases outer membrane-specific lipoproteins. Inner membrane-specific lipoproteins remain in the inner membrane because they each have a LolCDE-avoidance signal and therefore are not released by LolCDE. Only the LolC(A40P) mutation was previously found to cause outer membrane localization of lipoproteins despite their inner membrane-retention signals. Here, we isolated several new LolCDE mutants that cause outer membrane localization of lipoproteins possessing LolCDE-avoidance signals. Mutations were found in all three subunits of LolCDE, including the cytoplasmic ATPase subunit LolD. However, the extent of outer membrane sorting of inner membrane-specific lipoproteins differed depending on the mutation. Based on these observations, the molecular events underlying the recognition of lipoproteins by the LolCDE complex are discussed.