What cellular structure is primarily involved in degrading misfolded or damaged proteins?

The proteasome is the primary cellular structure responsible for degrading misfolded or damaged proteins through a process known as ubiquitin-proteasome degradation. Proteins that are improperly folded or marked for destruction are tagged with a small protein called ubiquitin, which signals them for degradation. The proteasome then recognizes these ubiquitinated proteins and unfolds them, allowing them to be translocated into its catalytic core, where they are broken down into small peptides.

This mechanism is essential for maintaining cellular protein homeostasis and preventing the accumulation of potentially toxic aggregates that can arise from dysfunctional proteins. The proteasome ensures that only properly folded and functional proteins are present in the cell, contributing to various cellular processes such as signaling, cell cycle control, and responses to cellular stress.

In contrast, the lysosome is involved in the degradation of larger biomolecules and cellular debris through autophagy and other pathways, while the endoplasmic reticulum plays a critical role in folding newly synthesized proteins and detecting misfolded proteins, which it may attempt to refold or ultimately signal for degradation via ubiquitin tagging. The mitochondrion primarily functions in energy production and metabolic processes rather than direct protein degradation.