Bacterial lipoproteins are anchored to membranes through a lipid moiety attached to the N-terminal Cys. Escherichia coli possesses more than 90 species of lipoproteins, most of which are localized in the outer membrane and others in the inner membrane. Sorting of lipoproteins to the outer membrane requires the Lol system comprising five Lol proteins. An ATP-binding cassette transporter, LolCDE, initiates the lipoprotein sorting by mediating the detachment of outer membrane-specific lipoproteins from the inner membrane. LolCDE does not recognize lipoproteins possessing Asp at position 2, which therefore remain anchored to the inner membrane. We will discuss the mechanism of LolCDE based on data obtained through in vitro experiments.