Single protein folding.
The development of single molecule force-clamp spectroscopy introduces a novel way to probe the dynamics of proteins by measuring their length and mechanical stability during each stage of folding. We use the the force-quench technique to first unfold and extend a protein and then we quench the force allowing the protein to collapse and then fold. This technique easily separates three distinct states during folding under force: the extended conformation,a weakly stable collapsed conformation corresponding to a "molten globule" and the native mechanically stable conformation. The collapse trajectory of an extended protein to its collapsed molten globule state is complex and governed by the recently discovered entropic barrier that forms when force is applied to a molecule. Once the molten globule state is reached, the protein needs to remain undisturbed for a surprisingly long time (seconds) to fully regain its native mechanically stable conformation. Mechanical stability is an excellent proxy for the distinct stages of folding. Our studies show that the physics of a folding protein can be accurately studied using force-clamp spectroscopy. Once the Physics of a folding protein is known, it should be possible to unify all available experimental observations, including bulk and single molecules. Current projects include an effort to identify the mechanisms by which homo-polypeptide expansions lead to misfolding of the host, triggering neuro-degeneration. Other projects are aimed at studying the folding pathways of the mechano-sensing protein talin (in collaboration with Dr. Mike Sheetz), various proteins of the extra-cellular matrix and of the giant muscle protein titin.