Fast Chain Contraction during Protein Folding: “Foldability” and Collapse Dynamics

Theory indicates that at least some proteins will undergo a rapid and unimpeded collapse, like a disorganized hydrophobic chain, prior to folding. Yet experiments continue to find signs of an organized, or barrier-limited, collapse in even the fastest ($\sim \mu \mathrm{s}$) folding proteins. Does the kinetic barrier represent a signature of the equilibrium “foldability” of these molecules? We have measured the rate of chain contraction in two nonfolding analogs of a very fast-collapsing protein. We find that these chains contract on the same time scale ($\sim {10}^{-5}\mathrm{s}$) as the natural protein, and both pass over an energetic barrier at least as large as that encountered by the protein. The equilibrium foldability of the native structure therefore does not alone determine the dynamics of collapse; even the disordered chains contract $\sim 1000\times$ slower than expected for an ideal chain.