Kinetics of folding and binding of an intrinsically disordered protein: The inhibitor of yeast aspartic proteinase YPrA

The 68 residue peptide IA(3) is an intrinsically unstructured protein that serves as an endogenous inhibitor of the yeast aspartic proteinase A (YPrA). Although unstructured in free solution, IA(3) forms an N-terminal alpha helix as it binds to YPrA, leading to subnanomolar inhibition of the protease. Equilibrium structural and inhibition studies provide little insight into the mechanism and kinetics of the coupled folding and binding interaction. We have used laser temperature jump spectroscopy to study the kinetics of folding of free IA(3) and of the interaction between IA(3) and YPrA. Inducing folding with trifluoroethanol cosolvent allows us to determine the folding rate (k(f) approximate to 0.3 (mu s)(-1)) and the unfolding rate (k(u) approximate to 3 (mu s)(-1)) for free IA(3) in water at 25 degrees C. A substantially faster relaxation process is observed in the presence of the proteinase; this process appears to be the kinetic signature of an intermediate binding step in the coupled folding and binding interaction of IA(3) and YPrA.