[NMR paper] Probing the transient dark state of substrate binding to GroEL by relaxation-based solution NMR.

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Probing the transient dark state of substrate binding to GroEL by relaxation-based solution NMR.

Probing the transient dark state of substrate binding to GroEL by relaxation-based solution NMR.

Proc Natl Acad Sci U S A. 2013 Jun 24;

Authors: Libich DS, Fawzi NL, Ying J, Clore GM


Abstract
The mechanism whereby the prototypical chaperonin GroEL performs work on substrate proteins has not yet been fully elucidated, hindered by lack of detailed structural and dynamic information on the bound substrate. Previous investigations have produced conflicting reports on the state of GroEL-bound polypeptides, largely due to the transient and dynamic nature of these complexes. Here, we present a unique approach, based on combined analysis of four complementary relaxation-based NMR experiments, to probe directly the "dark" NMR-invisible state of the model, intrinsically disordered, polypeptide amyloid ? (A?40) bound to GroEL. The four NMR experiments, lifetime line-broadening, dark-state exchange saturation transfer, relaxation dispersion, and small exchange-induced chemical shifts, are dependent in different ways on the overall exchange rates and populations of the free and bound states of the substrate, as well as on residue-specific dynamics and structure within the bound state as reported by transverse magnetization relaxation rates and backbone chemical shifts, respectively. Global fitting of all the NMR data shows that the complex is transient with a lifetime of