Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 110, Issue 32, Pages E2958-E2966Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1311996110
Keywords
protein machine; allostery; chaperonin
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Funding
- National Center for Research Resources from the National Institutes of Health [5P41RR015301-10]
- National Institute of General Medical Sciences from the National Institutes of Health [8 P41 GM103403-10]
- US Department of Energy [DE-AC02-06CH11357]
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The chaperonin proteins GroEL and GroES are cellular nanomachines driven by the hydrolysis of ATP that facilitate the folding of structurally diverse substrate proteins. In response to ligand binding, the subunits of a ring cycle in a concerted manner through a series of allosteric states (T, R, and R ''), enabling work to be performed on the substrate protein. Removing two salt bridges that ordinarily break during the allosteric transitions of the WT permitted the structure of GroEL-ADP in the R state to be solved to 2.7 angstrom resolution. Whereas the equatorial domain displays almost perfect sevenfold symmetry, the apical domains, to which substrate proteins bind, and to a lesser extent, the intermediate domains display a remarkable asymmetry. Freed of inter-subunit contacts, the apical domain of each subunit adopts a different conformation, suggesting a flexibility that permits interaction with diverse substrate proteins. This result contrasts with a previous cryo-EM study of a related allosteric ATP-bound state at lower resolution. After artificially imposing sevenfold symmetry it was concluded that a GroEL ring in the R-ATP state existed in six homogeneous but slightly different states. By imposing sevenfold symmetry on each of the subunits of the crystal structure of GroEL-ADP, we showed that the synthetic rings of (X-ray) GroEL-ADP and (cryo-EM) GroEL-ATP are structurally closely related. A deterministic model, the click stop mechanism, that implied temporal transitions between these states was proposed. Here, however, these conformational states are shown to exist as a structurally heterogeneous ensemble within a single ring.
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