The life cycle of protein

The life cycle of a typical protein begins with its synthesis on a ribosome. As the polypeptide chain grows, molecules of a chaperone protein bind along its length. This prevents misfolding of the nascent polypeptide. ATP binding causes chaperone release. For most proteins, the polypeptide then folds into a native conformation. However, for some proteins, like actin, an additional folding step occurs. The unfolded polypeptide is bound in a large chaperonin protein. Within the chaperonin, the polypeptide folds and is expelled from the complex. This process requires ATP. The newly liberated actin is then able to polymerize into a filament that becomes part of the actin cytoskeleton. The lifespan of a protein is determined by the amino acid at its N-terminus or by the presence of a destruction sequence, such as PEST. Protein degradation begins when ubiquitin is covalently attached to the target protein by a conjugating enzyme. Multiple rounds of this reaction modifies the protein with a ubiquitin chain.The chain targets the protein for the 26S proteasome, the major protein degradation machine in the cytoplasm. The proteasome cleaves the tagged protein into ubiquitin and short peptides.

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