title: Multiple cycles of global unfolding of GroEL-bound cyclophilin A evidenced by NMR.
abstract: GroE, the chaperonin system of Escherichia coli, prevents the aggregation of partially folded or misfolded proteins by complexing them in a form competent for subsequent folding to the native state. We examined the exchange of amide protons of cyclophilin A (CypA) interacting with GroEL, using NMR spectroscopy. We have applied labeling pulses in H2O to the deuterated GroEL-CypA-complex. When ATP and GroES were added after the labeling pulse, refolding of CypA could be accelerated to rates comparable to the amide proton exchange. This allowed the calculation of protection factors (PF) for the backbone amide protons in the GroEL-bound substrate protein. A set of highly protected protons in the native state (PF 10(5) to 10(7)) was observed to be much less protected (PF 10(2) to 10(4)) in complex with GroEL and, in contrast to the native structure, the protection factors were found to be quite uniform along the sequence suggesting that CypA with native-like structure undergoes multiple cycles of unfolding while bound to GroEL, which are faster than unfolding in free solution. Because of the small sequence dependence of the protection factors, unfolding must be global, and in this way the chaperone appears to resolve off-pathway intermediates and to support protein folding by annealing. Although in the complex with GroEL native-like states still predominate over globally unfolded states, this equilibrium is shifted 10(2) to 10(4)-fold toward the unfolded state when compared to CypA in free solution. Repeated global unfolding may be a key step in achieving a high yield of correctly folded proteins.
authors.affiliation: Biochemisches Institut der Universität Zürich, Winterthurerstr., Zürich, 190 8057, Switzerland.
authors.forename: S E
authors.initials: SE
authors.lastname: Nieba-Axmann
authors.forename: M
authors.initials: M
authors.lastname: Ottiger
authors.forename: K
authors.initials: K
authors.lastname: Wüthrich
authors.forename: A
authors.initials: A
authors.lastname: Plückthun
mesh_terms: D000255:Adenosine Triphosphate / Q000737:chemistry; D000591:Amino Acid Isomerases / Q000737:chemistry* / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism; D018835:Chaperonin 10 / Q000737:chemistry; D018834:Chaperonin 60 / Q000378:metabolism*; D004926:Escherichia coli / Q000737:chemistry; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D019696:Peptidylprolyl Isomerase; D011485:Protein Binding; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding
chemical_list: D002352:Carrier Proteins; D018835:Chaperonin 10; D018834:Chaperonin 60; D000255:Adenosine Triphosphate; D000591:Amino Acid Isomerases; D019696:Peptidylprolyl Isomerase