A nanomachine for getting proteins into working shape
Clare et al., ATP-Triggered Conformational Changes Delineate Substrate-Binding and -Folding Mechanics of the GroEL Chaperonin. Cell (2012), doi:10.1016/j.cell.2012.02.047
Researchers at Birkbeck Collegehave analysed thousands of images to recreate the elaborate movements of a protein “pot” that encapsulates other proteins and helps them find the right structure and remain healthy when exposed to stresses, e.g. elevated temperature or toxins such as alcohol.
Every cell has a variety of proteins known as molecular chaperones, which, as the name suggests, specialise on the guidance and protection of the innocent and vulnerable among their fellow proteins and shield them from undesirable interactions. Among these, one of the best studied is the bacterial GroEL-GroES system which takes the shape of two cylindrical pots glued back-to-back (each pot is a ring of seven molecules of GroEL) and a matching pair of lids (each made of a ring of seven molecules of GroES).
Previous research had shown that partially structured protein molecules undergo cycles of binding and release inside the pot until they emerge with the correct structure (identified by the fact that its surface no longer displays any of the oil-like water-repellent chemical groups that should be tucked away in the core of the structure). Structural analysis had also shown dramatic differences between the conformations that the GroEL molecules in the sides of each pot adopt in various functional states, e.g. with and without a protein inside, or with and without the GroES lid. Just how these different shapes interconvert remained a mystery, however.
Now Helen Saibil and her coworkers at Birkbeck College, collaborating with Art Horwich at Yale University and using the automated electron microscopy facility at the Scripps Research Institute in La Jolla, have crowned their long-running structural work on molecular chaperones using electron microscopy at very low temperatures (cryo-EM) by combining a large number of snapshots of GroEL-GroES complexes to build a movie that shows a plausible way in which the seven subunits of each pot tilt, expand, and twist to change their shapes during the functional cycle of this chaperone.
The movies (which will be available as supplementary information on the Cell web site) show several distinct phases of coordinated movement carried out by the seven subunits in the same ring, much like a synchronised swimming team. The movements they describe suggest how the subunits can first bind the protein inside the cavity, potentially stretch it to undo incorrectly folded molecules, and then release it to allow it to find its own structure in the aqueous environment inside the pot. If the protein still displays water-repellent patches after it is released back into solution, it will bind to a new GroEL ring and run through the cycle again. Once it is correctly folded it will be stable in the cell environment and will carry out its biological function.
“The operation of chaperone machines is of fundamental importance in the processes that underlie protein quality control and repair,” says Helen Saibil. “It is these functions that decline at different rates in different people during ageing. If we can understand the details of how these machines work and what determines their level of activity, we may arrive at a better understanding of age-related, degenerative diseases for which effective treatments are currently lacking.”
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