Final answer:
The anaphase-promoting complex/cyclosome (APC/C) ubiquitylates target proteins, marking them for proteolytic degradation by the proteasome. Ubiquitin acts as a flag indicating that a protein's lifespan is complete, and the specificity of proteolysis is controlled by ubiquitin ligases. This pathway regulates gene expression and maintains cellular homeostasis by degrading old, damaged, or surplus proteins.
Step-by-step explanation:
The anaphase-promoting complex/cyclosome (APC/C) is involved in the ubiquitin-proteasome pathway and is critical for the cell’s regulatory processes, such as the cell cycle. One function of the APC/C is to ubiquitylate target proteins, marking them for degradation. The mechanism for this process begins with a protein that needs to be destroyed, often because it is old, damaged, or otherwise no longer needed by the cell. This protein becomes polyubiquitinated when multiple molecules of a small protein called ubiquitin attach to it, effectively flagging it for removal.
The polyubiquitinated protein is then recognized by the proteasome, a large proteolytic complex. Within the proteasome, the target protein undergoes proteolytic degradation, being broken down into short peptides and ultimately into free amino acids, which are released back into the cytoplasm. The specificity of which proteins are targeted by the APC/C is largely determined by the ubiquitin ligases, especially the E3 ubiquitin ligase, which ensures that proteins are marked accurately and efficiently for degradation.
The ubiquitin-proteasomal degradation pathway is involved not only in protein turnover but also in regulating gene expression and responding to cellular stress. By controlling the degradation of specific proteins, the cell can alter protein content swiftly in response to changing conditions, which is fundamental for maintaining cellular homeostasis.