Kinetics of Folding of Leucine Zipper Domains

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Leucine zippers are short coiled coils frequently found in transcription factors where they serve as dimerization domains. The basic features contributing to the thermodynamic stability of leucine zippers are well understood, but very little is known about their folding kinetics. Leucine zippers have a simple and well defined structure and are, therefore, excellent models for the study of the concerted folding and assembly of polypeptide chains. Here we report on a fluorescence stopped flow investigation of the kinetics of association and dissociation of a series of model leucine zippers based on the common sequence Xzero-EYEALEKKLAAX1EAKX2QALEKKLEALEHG-amide (Xzero = N alpha-acetyl, N alpha-fluorescein-GGG, or N alpha-dimethylaminocoumarin-GGG; Xl = Leu or Ala; X2 = Leu, Ala, or Asn). When Xzero is fluorescein, self-quenching between adjacent fluorophores leads to a decrease in fluorescence emission intensity whereas unfolding of the coiled coil leads to an increase. In a heteromeric coiled coil containing both fluorophores, resonance energy transfer between the donor coumarin and the acceptor fluorescein is observed, and the mixing of labeled and nonlabeled peptides allows the measurement of the rates of strand exchange between leucine zippers. Exchange rates do not depend on peptide concentration, indicating that strand exchange is governed by the rate of dissociation of the coiled coil. Strand exchange between leucine zippers with X1 and X2 = Leu occurs with a half-time of approximately 30 min. A single Leu/Ala substitution at X1 or X2 decreases the half-time to approximately 1 s. Folding was also studied in a relaxation experiment in which a preexisting equilibrium between monomeric chains and coiled coils was rapidly disturbed by dilution with buffer, and the relaxation to the new equilibrium was followed by the increase in fluorescence. In peptides with X1, X2 = Ala or X1 = Ala, X2 = Asn the folding process can be described by a simple two-state monomer<-->dimer equilibrium with k(on) approximately 4 x 10(6) M-1 s-1 and k(off) approximately 10 s-1. Kd = k(off)/k(on) approximately 2.5 microM is in good agreement with the value of Kd obtained from equilibrium measurements. The peptides with a single Ala at X1 or X2 exhibit biphasic folding kinetics. One phase is concentration dependent and the other apparently concentration independent. This behavior can be interpreted as a monomer<-->dimer equilibrium coupled to an equilibrium between different conformational isomers. Leu to Ala and Leu to Asn substitutions in the hydrophobic core alter the folding kinetics in a position-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)