Chaperone function of FkpA, a heat shock prolyl isomerase, in the periplasm of Escherichia coli

The nature of molecular chaperones in the periplasm of Escherichia coli that assist newly translocated proteins to reach their native state has remained poorly defined. Here, we show that FkpA, a heat shock periplasmic peptidyl‐prolyl cis/trans isomerase (PPIase), suppresses the formation of inclusion bodies from a defective‐folding variant of the maltose‐binding protein, MalE31. This chaperone‐like activity of FkpA, which is independent of its PPIase activity, requires a full‐length structure of the protein. In vitro, FkpA does not catalyse a slow rate‐limiting step in the refolding of MalE31, but prevents its aggregation at stoichiometric amounts and promotes the reactivation of denaturated citrate synthase. We propose that FkpA functions as a chaperone for envelope proteins in the bacterial periplasm.