Ability of MBP or RBP signal peptides to influence folding and in vitro translocation of wild-type and hybrid precursors

Influence of tat mutations on the ribose-binding protein translocation in Escherichia coli

Proteins are exported across the bacterial cytoplasmic membrane either as unfolded precursors via the Sec machinery or in folded conformation via the Tat system. The ribose-binding protein (RBP) of Escherichia coli is a Sec-pathway substrate. Intriguingly, it exhibits fast folding kinetics and its export is independent of SecB, a general chaperone protein dedicated for protein secretion. In this study, we found that the quantity of RBP was significantly reduced in the periplasm of tat mutants, which was restored by in trans expression of the tatABC genes. Pulse-chase experiments showed that significant amount of wild-type RBP was processed in a secY mutant in the presence of azide (SecA inhibitor), whereas the processing of a slow folding RBP derivative was almost completely blocked under the same conditions. These results would suggest that under the Sec-defective conditions the export of a portion of folded RBP could be rescued by the Tat system.

Folded HasA inhibits its own secretion through its ABC exporter

Gram-negative bacterial proteins secreted by ABC exporters carry a secretion signal in their carboxylic extremities. This characteristic suggests that the polypeptide needs to be fully synthesized before it can be secreted and, therefore, presumably may fold at least in part before its secretion. We investigated the relationship between folding and secretion using HasA, a hemoprotein of Serratia marcescens secreted into the extracellular medium by a dedicated Has ABC exporter. We first demonstrated that when HasA is sequestered in the cytoplasm it can acquire its tertiary structure, as assessed from its capacity to bind heme. The cytoplasmic pool of HasA cannot be secreted and inhibits the secretion of newly synthesized molecules. HasA folding in the cytoplasm was independent of either its capacity to bind heme or the presence of SecB, although SecB is essential for HasA secretion. Our findings indicate a strong coupling between synthesis and secretion in the type I secretion pathway.

Escherichia coli SecB stimulates export without maintaining export competence of ribose-binding protein signal sequence mutants

Ribose-binding protein (RBP) is exported to the periplasm of Escherichia coli via the general export pathway. An rbsB-lacZ gene fusion was constructed and used to select mutants defective in RBP export. The spontaneous Lac+ mutants isolated in this selection contained either single-amino-acid substitutions or a deletion of the RBP signal sequence. Intact rbsB genes containing eight different point mutations in the signal sequence were reconstructed, and the effects of the mutations on RBP export were examined. Most of the mutations caused severe defects in RBP export. In addition, different suppressor mutations in SecY/PrlA protein were analyzed for their effects on the export of RBP signal sequence mutants in the presence or absence of SecB. Several RBP signal sequence mutants were efficiently suppressed, but others were not suppressed. Export of an RBP signal sequence mutant in prlA mutant strains was partially dependent on SecB, which is in contrast to the SecB independence of wild-type RBP export. However, the kinetics of export of an RBP signal sequence mutant point to a rapid loss of pre-RBP export competence, which occurs in strains containing or lacking SecB. These results suggest that SecB does not stabilize the export-competent conformation of RBP and may affect translocation by stabilizing the binding of pre-RBP at the translocation site.