The SecB chaperone is bifunctional in Serratia marcescens: SecB is involved in the Sec pathway and required for HasA secretion by the ABC transporter.
HasA is the secreted hemophore of the heme acquisition system (Has) of Serratia marcescens. It is secreted by a specific ABC transporter apparatus composed of three proteins: HasD, an inner membrane ABC protein; HasE, another inner membrane protein; and HasF, a TolC homolog. Except for HasF, the structural genes of the Has system are encoded by an iron-regulated operon. In previous studies, this secretion system has been reconstituted in Escherichia coli, where it requires the presence of the SecB chaperone, the Sec pathway-dedicated chaperone. We cloned and inactivated the secB gene from S. marcescens. We show that S. marcescens SecB is 93% identical to E. coli SecB and complements the secretion defects of a secB mutant of E. coli for both the Sec and ABC pathways of HasA secretion. In S. marcescens, SecB inactivation affects translocation by the Sec pathway and abolishes HasA secretion. This demonstrates that S. marcescens SecB is the genuine chaperone for HasA secretion in S. marcescens. These results also demonstrate that S. marcescens SecB is bifunctional, as it is involved in two separate secretion pathways. We investigated the effects of secB point mutations in the reconstituted HasA secretion pathway by comparing the translocation of a Sec substrate in various mutants. Two different patterns of SecB residue effects were observed, suggesting that SecB functions may differ for the Sec and ABC pathways.
Sapriel
Guillaume
G
Unité des Membranes Bactériennes, URA CNRS 2172, Institut Pasteur, 25-28 rue du Dr. Roux, 75724 Paris Cedex 15, France.
Wandersman
Cécile
C
Delepelaire
Philippe
P
D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins; D003001:Cloning, Molecular; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D017353:Gene Deletion; D005816:Genetic Complementation Test; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D018832:Molecular Chaperones / Q000378:metabolism*; D008969:Molecular Sequence Data; D017354:Point Mutation; D012706:Serratia marcescens / Q000235:genetics / Q000378:metabolism* / Q000502:physiology
D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D002352:Carrier Proteins; C093956:HasA protein, Serratia marcescens; D008565:Membrane Proteins; D018832:Molecular Chaperones; C057842:SecB protein, Bacteria