Phosphoglycerate Kinase Can Adopt a Topologically Misfolded Form that is More Stable than its Native State.

The native states of globular proteins are typically viewed as being the most stable conformations on their respective proteins' soluble free energy landscapes. This view, known as the Thermodynamic Hypothesis, explains why many proteins can reversibly refold after being denatured. Here we report an intriguing counterexample to this paradigm. When E. coli phosphoglycerate kinase (PGK) is stimulated to refold upon dilution from denaturant, instead of returning to its native state, it populates an unusual misfolded form that is monomeric and native-like, but which is even more kinetically stable than its native form, as based on its resistance to thermal and detergent-induced denaturation. Moreover, this misfolded form cannot self-correct, even for days. We show that the key structural feature of this misfolded form of PGK is topological in nature by demonstrating that kinetically stable misfolded forms do not form any longer if PGK is circularized, which prevents its termini from threading through other portions of the protein. Our findings demonstrate that a misfolded protein need not aggregate or form an amyloid to become stabilized with respect to the native state, and call attention to topologically-misfolded proteins as a potential Achilles heel to the cellular proteostasis network.