The bacterial lipocalins.
The lipocalins were once regarded as a eukaryotic protein family, but new members have been recently discovered in bacteria. The first bacterial lipocalin (Blc) was identified in Escherichia coli as an outer membrane lipoprotein expressed under conditions of environmental stress. Blc is distinguished from most lipocalins by the absence of intramolecular disulfide bonds, but the presence of a membrane anchor is shared with two of its closest homologues, apolipoprotein D and lazarillo. Several common features of the membrane-anchored lipocalins suggest that each may play an important role in membrane biogenesis and repair. Additionally, Blc proteins are implicated in the dissemination of antibiotic resistance genes and in the activation of immunity. Recent genome sequencing efforts reveal the existence of at least 20 bacterial lipocalins. The lipocalins appear to have originated in Gram-negative bacteria and were probably transferred horizontally to eukaryotes from the endosymbiotic alpha-proteobacterial ancestor of the mitochondrion. The genome sequences also reveal that some bacterial lipocalins exhibit disulfide bonds and alternative modes of subcellular localization, which include targeting to the periplasmic space, the cytoplasmic membrane, and the cytosol. The relationships between bacterial lipocalin structure and function further illuminate the common biochemistry of bacterial and eukaryotic cells.
Bishop
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Department of Laboratory Medicine and Pathobiology, Faculty of Medicine, University of Toronto, 6213 Medical Sciences Building, Toronto, Ont. M5S 1A8, Canada. russell.bishop@utoronto.ca
D000595:Amino Acid Sequence; D001053:Apolipoproteins / Q000378:metabolism; D053399:Apolipoproteins D; D001425:Bacterial Outer Membrane Proteins / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D004220:Disulfides / Q000378:metabolism; D004352:Drug Resistance, Microbial / Q000502:physiology; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins; D054834:Lipocalins; D008074:Lipoproteins / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D008969:Molecular Sequence Data; D017386:Sequence Homology, Amino Acid; D013347:Subcellular Fractions; D015533:Transcriptional Activation
D001053:Apolipoproteins; D053399:Apolipoproteins D; D001425:Bacterial Outer Membrane Proteins; D004220:Disulfides; D029968:Escherichia coli Proteins; D054834:Lipocalins; D008074:Lipoproteins; C095894:blc protein, E coli