Protein folding principles: Foldons, sequential stabilization, and optional errors

BIOT 23

Krishna Mallela and S. Walter Englander. Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, 1007 Stellar Chance Bldg., 422 Curie Blvd, Philadelphia, PA 19104-6059
Detailed structural information from hydrogen exchange and related experiments indicate the following three physical principles underlying protein folding pathways. (1) Proteins are made up of small cooperative unfolding/folding submolecular units known as foldons. (2) Proteins construct these foldon pieces to progressively build their final native states using the sequential stabilization principle where pre-formed foldons guide and template the subsequent foldons. (3) Ubiquitous optional misfolding errors can corrupt different naturally occurring on-pathway intermediates and cause intermediates to accumulate by inserting error-repair barriers at different points along the pathway. The first two principles dictate that the folding pathway of a protein gets predetermined by its component foldon substructure, and the order of steps is set by the way the foldon units are organized in the native structure. The third principle dictates whether the pathway appears to be kinetically 2-state or multi-state or heterogeneous. The integration of these three well-documented principles into a coherent mechanism provides a unifying explanation for how proteins fold and why they fold in that way.