The outer membrane-specific lipoproteins of Escherichia coli are released from the inner membrane as a water-soluble complex with LolA and then transferred to the outer membrane receptor, LolB. LolA thus plays a critical role in the sorting and outer membrane localization of lipoproteins. To dissect the LoIA function, the highly conserved residues were subjected to random mutagenesis, followed by selection for a growth defect. LolA(R43L), one of mutants thus constructed, possessed Leu in place of Arg at position 43 and caused accumulation of the LolA(R43L)-lipoprotein complex in the periplasm. LolA(R43L) was as active as wild-type LolA as to the release of lipoproteins from spheroplasts. In marked contrast, the transfer of lipoproteins from LolA(R43L) to LolB was completely inhibited, indicating that Arg at position 43 of LoIA is involved in the lipoprotein transfer reaction.