Peptidyl prolyl cis-trans isomerases can enzymatically assist
protein folding, but these enzymes exclusively target the peptide bond
preceding proline residues. Here we report the identification of the Hsp70
chaperone DnaK as the first member of a novel enzyme class of secondary amide
peptide bond cis-trans isomerases (APIases). APIases selectively
accelerate the cis-trans isomerization of nonprolyl peptide bonds.
Results from independent experiments support the APIase activity of DnaK: (i)
exchange crosspeaks between the cis-trans conformers appear in 2D
1H NMR exchange spectra of oligopeptides (ii) the rate constants
for the cis-trans isomerization of various dipeptides increase and (iii)
refolding of the RNase T1 P39A variant is catalyzed. The APIase activity shows
both regio and stereo selectivity and is stimulated two-fold in the presence of
the complete DnaK/GrpE/DnaJ/ATP refolding system. Moreover, known DnaK-binding
oligopeptides simultaneously affect the APIase activity of DnaK and the
refolding yield of denatured firefly luciferase in the presence of
DnaK/GrpE/DnaJ/ATP. These results suggest a new role for the chaperone as a
regioselective catalyst for bond rotation in polypeptides.
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