Secretion of proteins with dimerization capacity by the haemolysin type I transport system of Escherichia coli.
The tolerance of the haemolysin transport system (Hly) for exporting dimeric protein substrates to the supernatants of Escherichia coli cultures was examined. A strong dimerization domain (i.e. an amphipathic alpha-helix capable of forming a leucine zipper in the yeast transcription factor GCN4) was inserted into an epitope-tagged version of the 23 kDa C-terminal secretion signal of haemolysin (EHlyA). The zipper-containing polypeptide (ZEHlyA) was effectively secreted by E. coli cells carrying the HlyBD transporter and accumulated in the culture media as a stable dimer as determined by gel filtration chromatography. In vivo protein cross-linking experiments and coexpression with a secretion-deficient derivative of ZEHlyA indicated that leucine zipper-dependent dimerization occurs following secretion. To test whether dimerization allows the correct folding of the secreted polypeptide, immunoglobulin V(HH)-domains obtained from camel antibodies were fused to EHlyA and ZEHlyA. Functional dimerization of the ZEHlyA hybrid was anticipated to increase the apparent binding affinity (i.e. avidity) of the V(HH) moiety, thus becoming an excellent reporter of correct protein folding and dimerization. Both V(HH)-EHlyA and V(HH)-ZEHlyA hybrids were quantitatively secreted and found in the extracellular medium as active monomers and dimers respectively. When compared with their monomeric counterparts, the dimeric V(HH)-ZEHlyA molecules showed superior binding properties to their cognate antigen, with a 10-fold increase in their avidity. These data reveal a non-anticipated permissiveness of the Hly type I transport machinery for the secretion of substrates with dimerization capacity.
Centro Nacional de Biotecnología del Consejo Superior de Investigaciones Científicas (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain.
Sofía
S
Fraile
Amalia
A
Muñoz
Víctor
V
de Lorenzo
Luis Angel
LA
Fernández
D000595:Amino Acid Sequence; D001692:Biological Transport; D003470:Culture Media; D019281:Dimerization; D004926:Escherichia coli / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D006460:Hemolysin Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016350:Leucine Zippers; D008969:Molecular Sequence Data; D017510:Protein Folding
D003470:Culture Media; D029968:Escherichia coli Proteins; D006460:Hemolysin Proteins; C036507:Hlya protein, E coli