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 cob 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 LoIC(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 sub units 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 (C) 2010 Elsevier Inc All rights reserved