Chaperone rings in protein folding and degradation

Chaperone rings play a vital role in the opposing ATP-mediated processes of folding and degradation of many cellular proteins, but the mechanisms by w hich they assist these life and death actions are only beginning to be unde rstood, Ring structures present an advantage to both processes, providing f or compartmentalization of the substrate protein inside a central cavity in which multivalent, potentially cooperative interactions can take place bet ween the substrate and a high local concentration of binding sites, while a ccess of other proteins to the cavity is restricted sterically. Such restri ction prevents outside interference that could lead to nonproductive fates of the substrate protein while it is present in non-native form, such as ag gregation. At the step of recognition, chaperone rings recognize different motifs in their substrates, exposed hydrophobicity in the case of protein-f olding chaperonins, and specific "tag" sequences in at least some cases of the proteolytic chaperones. For both folding and proteolytic complexes, ATP directs conformational changes in the chaperone rings that govern release of the bound polypeptide. In the case of chaperonins, ATP enables a release d protein to pursue the native state in a sequestered hydrophilic folding c hamber, and, in the case of the proteases, the released polypeptide is tran slocated into a degradation chamber. These divergent fates are at least par tly governed by very different cooperating components that associate with t he chaperone rings: that is, cochaperonin rings on one hand and proteolytic ring assemblies on the other, Here we review the structures and mechanisms of the two types of chaperone ring system.