How the cell chooses between refolding and degrading
A misfolded protein is first sampled by chaperones. If a chaperone can bind and the client can be released in a folding-competent state, repeated cycles of binding and release give the polypeptide a chance to reach the native conformation. If the client remains persistently misfolded, or if it exposes surfaces that chaperones cannot shield, it becomes a substrate for ubiquitination. The E3 ligases that recognize misfolded proteins are the decision point: their activity determines whether the client is tagged for degradation. If the proteasome is impaired or the client is aggregation-prone, the cell can route the cargo to autophagy. The choice is therefore not binary but a hierarchy: refold first, degrade second, and if degradation fails, aggregate.
Consequences of degradation failure
If the proteasome is inhibited, ubiquitinated proteins accumulate, and the cell loses the ability to clear short-lived regulatory proteins as well as misfolded ones. If autophagy is blocked, large aggregates and damaged organelles persist, and the cell cannot recycle the building blocks it needs under stress. In both cases, the immediate effect is a rising burden of non-functional protein. That burden is not inert: aggregates can sequester chaperones and transcription factors, disrupt membrane integrity, and interfere with transport. The cell may respond by inducing more chaperones and more degradation machinery, but if the load continues to rise, the response itself becomes a source of stress, and the cell moves toward dysfunction or death.
The threshold is not fixed
The point at which capacity is exceeded depends on the cell type, the nature of the misfolded protein, and the duration of the stress. A secretory cell with a large endoplasmic reticulum has a different chaperone budget than a neuron. A short pulse of misfolding may be fully cleared, while the same total load delivered over hours may overwhelm the system. This is the same intensity-and-duration logic introduced in Chapter 1, now applied to a specific adaptive system.