The SecB Chaperone Is Bifunctional in Serratia marcescens : SecB Is Involved in the Sec Pathway and Required for HasA Secretion by the ABC Transporter

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Abstract
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.