The proteasome primarily monitors which type of proteins?

The proteasome is primarily responsible for the degradation of misfolded proteins, which are proteins that fail to adopt their proper three-dimensional structures. This misfolding can lead to the accumulation of potentially toxic aggregates within the cell, impairing cellular function. The proteasome recognizes these misfolded proteins, marking them for destruction to maintain protein quality control and cellular homeostasis.

While ubiquitinated proteins are indeed the primary substrates for degradation by the proteasome, the specific role of the proteasome centers around ensuring that misfolded proteins do not accumulate. Ubiquitination serves as a signal for degradation, but it is the misfolding of the proteins that is being directly monitored and addressed by the proteasome.

Functional proteins and folded proteins are not the primary focus of the proteasome's monitoring system as they are typically properly maintained within the cell. Therefore, the proteasome's role is essential in targeting specifically misfolded proteins that could disrupt cellular processes if allowed to persist.