Spontaneous refolding of GdnHCl denatured bovine carbonic anhydrase II (BCA II) shows at least three phases: a burst phase, a fast phase, and a slow phase. The fast and slow phases are both controlled by proline isomerization. However, we find that in trigger factor (TF)-assisted BCA II folding, only the fast phase is catalyzed by wild-type TF, suggesting that certain proline residues are accessible in folding intermediates. The refolding yields of BCA II assisted by wild-type TF and TF mutants which lack PPIase activity are about the same, which provides further experimental evidence that the PPIase and chaperone activities of TF are independent. The binding of TF to folding intermediates during BCA II refolding was characterized by chemical crosslinking and Western blotting. A scheme for TF-assisted BCA II folding is proposed and the possible role of the TF dimer as a "binding" chaperone in vivo is discussed. (C) 2003 Elsevier Inc. All rights reserved.