title: Green fluorescent protein functions as a reporter for protein localization in Escherichia coli.
abstract: The use of green fluorescent protein (GFP) as a reporter for protein localization in Escherichia coli was explored by creating gene fusions between malE (encoding maltose-binding protein [MBP]) and a variant of gfp optimized for fluorescence in bacteria (GFPuv). These constructs encode hybrid proteins composed of GFP fused to the carboxy-terminal end of MBP. Fluorescence was not detected when the hybrid protein was synthesized with the MBP signal sequence. In contrast, when the MBP signal sequence was deleted, fluorescence was observed. Cell fractionation studies showed that the fluorescent MBP-GFP hybrid protein was localized in the cytoplasm, whereas the nonfluorescent version was localized to the periplasmic space. Smaller MBP-GFP hybrid proteins, however, exhibited abnormal fractionation. Expression of the gene fusions in different sec mutants, as well as signal sequence processing assays, confirmed that the periplasmically localized hybrid proteins were exported by the sec-dependent pathway. The distinction between fluorescent and nonfluorescent colonies was exploited as a scorable phenotype to isolate malE signal sequence mutations. While expression of hybrid proteins comprised of full-length MBP did not result in overproduction lethality characteristic of some exported beta-galactosidase hybrid proteins, synthesis of shorter, exported hybrid proteins was toxic to the cells. Purification of MBP-GFP hybrid protein from the different cellular compartments indicated that GFP is improperly folded when localized outside of the cytoplasm. These results suggest that GFP could serve as a useful reporter for genetic analysis of bacterial protein export and of protein folding.
authors.affiliation: Department of Microbiology, Iowa State University, Ames 50011, USA.
authors.forename: B J
authors.initials: BJ
authors.lastname: Feilmeier
authors.forename: G
authors.initials: G
authors.lastname: Iseminger
authors.forename: D
authors.initials: D
authors.lastname: Schroeder
authors.forename: H
authors.initials: H
authors.lastname: Webber
authors.forename: G J
authors.initials: GJ
authors.lastname: Phillips
mesh_terms: D018528:ATP-Binding Cassette Transporters; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D001692:Biological Transport / Q000235:genetics; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002451:Cell Compartmentation; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins; D017930:Genes, Reporter; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism*; D058619:Maltose-Binding Proteins; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins; D009154:Mutation; D019897:Periplasm / Q000378:metabolism; D033902:Periplasmic Binding Proteins
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D008164:Luminescent Proteins; C461619:MalE protein, E coli; D058619:Maltose-Binding Proteins; D009004:Monosaccharide Transport Proteins; D033902:Periplasmic Binding Proteins; C461599:maltose transport system, E coli; D049452:Green Fluorescent Proteins