The process of protein ubiquitination is executed by a complex cellular machinery the protein to be degraded is specifically recognized by a ubiquitin-ligase enzyme (E3) that cooperates with one or more ubiquitin-conjugating enzymes (or E2s) to transfer multiple moieties of the 76-amino-acid protein ubiquitin to the substrate 2.įeldman, D. Molecular chaperones are nature's solution to these. This requires, in essence, a molecular quality control system surveying individual protein molecules in a promiscuous manner. Polypeptides that are altered by mutation or modification are targeted for degradation by covalent addition of polyubiquitin, which is subsequently recognized by the 26S proteasome 2. A large group of unrelated protein families whose role is to stabilize unfolded proteins, unfold them for translocation across membranes or for degradation, and. To make life resilient, factors are needed that buffer the negative side of protein flexibility and even expand the accessible conformational space for proteins. However, not all newly synthesized or stress-denatured proteins can successfully attain their native states. Indeed, cells greatly increase their concentrations of some chaperones in response to such conditions, hence the term 'heat-shock proteins' (HSPs). In addition, when proteins unfold as a result of cellular stress, chaperones protect them, prevent their aggregation, and restore them to the native state once the stress has subsided. Several classes of molecular chaperones ensure that newly synthesized proteins, which emerge from ribosomes in an extended conformation, reach the native state rapidly and efficiently. It is well established that molecular chaperones play a key role in assisting cellular protein folding 1.
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