title: Roles of the protruding loop of factor B essential for the localization of lipoproteins (LolB) in the anchoring of bacterial triacylated proteins to the outer membrane.
abstract: The Lol system comprising five Lol proteins, LolA through LolE, sorts Escherichia coli lipoproteins to outer membranes. The LolCDE complex, an ATP binding cassette transporter in inner membranes, releases outer membrane-specific lipoproteins in an ATP-dependent manner, causing formation of the LolA-lipoprotein complex in the periplasm. LolA transports lipoproteins through the periplasm to LolB on outer membranes. LolB is itself a lipoprotein anchored to outer membranes, although the membrane anchor is functionally dispensable. LolB then localizes lipoproteins to outer membranes through largely unknown mechanisms. The crystal structure of LolB is similar to that of LolA, and it possesses a hydrophobic cavity that accommodates acyl chains of lipoproteins. To elucidate the molecular function of LolB, a periplasmic version of LolB, mLolB, was mutagenized at various conserved residues. Despite the lack of acyl chains, most defective mutants were insoluble. However, a derivative with glutamate in place of leucine 68 was soluble and unable to localize lipoproteins to outer membranes. This leucine is present in a loop protruding from mLolB into an aqueous environment, and no analogous loop is present in LolA. Thus, leucine 68 was replaced with other residues. Replacement by acidic, but not hydrophobic, residues generated for the first time mLolB derivatives that can accept but cannot localize lipoproteins to outer membranes. Moreover, deletion of the leucine with neighboring residues impaired the lipoprotein receptor activity. Based on these observations, the roles of the protruding loop of LolB in the last step of lipoprotein sorting are discussed.
authors.affiliation: Institute of Molecular and Cellular Biosciences, University of Tokyo, Tokyo 113-0032, Japan.
authors.forename: Yumi
authors.initials: Y
authors.lastname: Hayashi
authors.affiliation: Institute of Molecular and Cellular Biosciences, University of Tokyo, Tokyo 113-0032, Japan.
authors.forename: Ryoji
authors.initials: R
authors.lastname: Tsurumizu
authors.affiliation: Institute of Molecular and Cellular Biosciences, University of Tokyo, Tokyo 113-0032, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Tsukahara
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
authors.forename: Kazuki
authors.initials: K
authors.lastname: Takeda
authors.affiliation: Department of Nutritional Sciences, University of Morioka, Takizawa 020-0694 Iwate, Japan.
authors.forename: Shin-Ichiro
authors.initials: SI
authors.lastname: Narita
authors.affiliation: Department of Nutritional Sciences, University of Morioka, Takizawa 020-0694 Iwate, Japan.
authors.forename: Makiko
authors.initials: M
authors.lastname: Mori
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
authors.forename: Kunio
authors.initials: K
authors.lastname: Miki
authors.affiliation: Department of Nutritional Sciences, University of Morioka, Takizawa 020-0694 Iwate, Japan. Electronic address: htokuda@morioka-u.ac.jp.
authors.forename: Hajime
authors.initials: H
authors.lastname: Tokuda
mesh_terms: D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism*; D005816:Genetic Complementation Test; D018698:Glutamic Acid / Q000378:metabolism; D007930:Leucine / Q000378:metabolism; D008074:Lipoproteins / Q000378:metabolism; D018832:Molecular Chaperones; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009154:Mutation; D019897:Periplasm / Q000378:metabolism; D033902:Periplasmic Binding Proteins / Q000378:metabolism; D010957:Plasmids / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013696:Temperature
chemical_list: D001425:Bacterial Outer Membrane Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D008074:Lipoproteins; C116111:LolA protein, E coli; C087012:LolB protein, E coli; D018832:Molecular Chaperones; D033902:Periplasmic Binding Proteins; D018698:Glutamic Acid; D007930:Leucine
keywords: Bacterial Lipoprotein; Crystal Structure; Escherichia coli; Lipoprotein; LolB; Membrane Proteins; Periplasm; Protein Export; Protein Sorting; Protein Structure