The fast and the slow: Folding and trapping of λ _6-85

Molecular dynamics simulations combining many microsecond trajectories have recently predicted that a very fast folding protein like lambda repressor fragment λ _6-85 D14A could have a slow millisecond kinetic phase. We investigated this possibility by detecting temperature-jump relaxation to 5 ms. While λ _6-85 D14A has no significant slow phase, two even more stable mutants do. A slow phase of λ _6-85 D14A does appear in mild denaturant. The experimental data and computational modeling together suggest the following hypothesis: λ _6-85 takes only microseconds to reach its native state from an extensively unfolded state, while the latter takes milliseconds to reach compact β-rich traps. λ _6-85 is not only thermodynamically but also kinetically protected from reaching such "intramolecular amyloids" while folding.