Membrane-associated DegP in Bordetella chaperones a repeat-rich secretory protein
Résumé
The chaperone/protease DegP belongs to the HtrA superfamily and is involved in protein quality control in the periplasm of Gram‐negative bacteria. In Escherichia coli, typical substrates are unfolded or misfolded globular proteins that trigger the rearrangement of inactive DegP hexamers into substrate‐sequestering 12‐ or 24‐mers ‘cages’ for refolding or degradation. In Bordetella pertussis, DegPBp facilitates, in addition, the secretion of FHA, a long β‐helical adhesin that passes through the periplasm in an extended conformation. We show that DegPBp exists as soluble trimers and as a membrane‐associated form. Different substrates interact differently with the distinct forms of DegPBp, and membrane‐associated DegPBp has high affinity for non‐native FHA. Unlike more globular substrates, FHA does not efficiently mediate rearrangement of trimers into proteolytically active, short‐lived dodecamers. In contrast to these dodecamers, membrane‐associated DegPBp is not committed to substrate degradation, although it is proteolytically competent. In B. pertussis, membrane‐associated DegPBp thus represents a specific functional form serving as a holding chaperone for client proteins including FHA. If FHA secretion is impaired, membrane‐associated DegPBp participates in its degradation. This form of DegPBp is appropriate to handle substrates unsuitable to be sequestered in cages or non‐folded, secretory proteins that must not be degraded.