title: Identification of a new porin, RafY, encoded by raffinose plasmid pRSD2 of Escherichia coli.
abstract: The conjugative plasmid pRSD2 carries a raf operon that encodes a peripheral raffinose metabolic pathway in enterobacteria. In addition to the previously known raf genes, we identified another gene, rafY, which in Escherichia coli codes for an outer membrane protein (molecular mass, 53 kDa) similar in function to the known glycoporins LamB (maltoporin) and ScrY (sucrose porin). Sequence comparisons with LamB and ScrY revealed no significant similarities; however, both lamB and scrY mutants are functionally complemented by RafY. Expressed from the tac promoter, RafY significantly increases the uptake rates for maltose, sucrose, and raffinose at low substrate concentrations; in particular it shifts the apparent K(m) for raffinose transport from 2 mM to 130 microM. Moreover, RafY permits diffusion of the tetrasaccharide stachyose and of maltodextrins up to maltoheptaose through the outer membrane of E. coli. A comparison of all three glycoporins in regard to their substrate selectivity revealed that both ScrY and RafY have a broad substrate range which includes alpha-galactosides while LamB seems to be restricted to malto-oligosaccharides. It supports growth only on maltodextrins but not, like the others, on raffinose and stachyose.
authors.affiliation: Arbeitsgruppe Genetik, Fachbereich Biologie/Chemie, Universität Osnabrück, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Ulmke
authors.forename: J W
authors.initials: JW
authors.lastname: Lengeler
authors.forename: K
authors.initials: K
authors.lastname: Schmid
mesh_terms: D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000235:genetics*; D001483:Base Sequence; D001692:Biological Transport; D003001:Cloning, Molecular; D004926:Escherichia coli / Q000235:genetics*; D029968:Escherichia coli Proteins; D005798:Genes, Bacterial; D008320:Maltose / Q000031:analogs & derivatives / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000378:metabolism; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins; D009844:Oligosaccharides / Q000378:metabolism; D010957:Plasmids; D018272:Porins / Q000235:genetics*; D011887:Raffinose / Q000378:metabolism; D011991:Receptors, Virus / Q000378:metabolism; D027981:Symporters
chemical_list: D001425:Bacterial Outer Membrane Proteins; D029968:Escherichia coli Proteins; C050221:LacY protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D009844:Oligosaccharides; D018272:Porins; C108304:RafY protein, E coli; D011991:Receptors, Virus; D027981:Symporters; C016532:maltoporins; D008320:Maltose; C021499:lactose permease; C009819:maltotetraose; D011887:Raffinose