Chaperonin's Iris-Like 'lid'

By twirling rather than flapping is how a "lid" on the barrel-shaped eukaryotic chaperonin called TRiC closes and opens when proteins enter or depart its interior (Nat. Struct. Mol. Biol., DOI: 10.1038/nsmb.1436). TRiC is a large host complex that provides a protected environment for guest proteins to use as a kind of private dressing room in which to fold properly. After a guest enters, TRiC's lid closes to confine the protein and give it time to fold. Judith Frydman of Stanford University; Wah Chiu of Baylor College of Medicine, Houston; Andrej Sali of the University of California, San Francisco; and coworkers used single-particle electron cryomicroscopy and protein modeling to determine the molecular mechanism of the lid's motion, which is powered by adenosine triphosphate (ATP). Researchers had previously speculated that the lid closed and opened like a flap, but the new findings surprisingly indicate that it closes and opens rotationally, like the iris of a camera lens, and that part of this rotational motion is translated into the interior of the chaperonin. The work also suggests how evolution allowed TRiC to diverge from prokaryotic chaperonins like GroEL, which has a separate detachable lid.