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Chaperone proteins differ in their structures and specific functions - but all of them help proteins fold. Some of them do this by sequestering nascent proteins by binding them (i.e., reducing the concentration of unbound chaperone bound transcripts) with various effects. Others are less subtle about it: they literally form a protective barrel into which unfolded or partially folded proteins are dumped, a lid clamps itself on the top and the proteins fold correctly inside that protective environment. Mostly it is proteins with hydrophobic interiors - also called "globular proteins" - which need chaperones to fold appropriately. Otherwise, the partially folded proteins would become trapped in energetically favourable but wrong conformations.
Example would be for the MHC receptor when being formed, chaperone molecules protect it from degradation untill a peptide is bound to it and becomes expressed on the cell surface
due to crowded environment of the cell proteins may form aggregates due to hydrophobic attraction between its surfaces specially proteins with large hydrophobic surface area so protein molecule enter chaprone which is a large cylindrical macromolecule to continue folding and burn its hydrophobic side chans in the core so no aggregation occur
excerpt from The Importance of Self-Assembly pp. 33-34 Becker's World of Cell. John Ellis and Van der Vies proposed that interactions that drive protein folding may need to be assisted and controlled by proteins known as molecular chaperones (chaperonins) to reduce the probability of formation of incorrect structures having no biological activity. Chaperonins are proteins that facilitate the correct assembly of proteins and protein-containing structures but are not components of the assembled structures. The molecular chaperones that have been identified to date do not convey information either for polypeptide folding or for the assembly of multiple polypeptides into a single protein. Instead, they function by binding to specific regions that are exposed only in the early stages of assembly, thereby inhib- iting unproductive assembly pathways that would lead to incorrect structures. Commenting on the term molecular chaperone, Ellis and Van der Vies observed that “the term chaperone is appropriate for this family of proteins because the role of the human chaperone is to prevent incorrect inter- actions between people, not to provide steric information for those interactions” (Ellis and Van der Vies, 1991, p. 323).