LolB, catalyzing the last step of lipoprotein transfer from the inner to the Outer membrane of Escherichia coli, is itself a lipoprotein anchored to the Outer membrane. Five Trp residues of LolB are conserved among LolB homologs in Gram-negative bacteria. These Trp residues were mutagenized to obtain defective LolB mutants. Mutation of Trp at position 52 to Pro impaired the receptor activity and caused accumulation of the LolA-lipoprotein complex in the periplasm. similar mutants were obtained for Trp at position 117. A mutant with Gly instead of Trp at position 148 retained the receptor activity but inhibited growth upon its overproduction. The outer membrane sorting of this mutant seemed to be defective, lipoprotein transfer thereby being perturbed when it was overproduced. Despite the strong conservation, no defective mutant for Trp at position 183 was obtained, and only weak mutants were isolated for Trp at position 18. Based on the crystal structure of LolB, the phenotypes of these mutants are discussed. (C) 2004 Elsevier Inc. All rights reserved.