Question

How are misfolded proteins and incompletely assembled proteins retained in the ER?

A. chaperone proteins bind them and prevent their entry into vesicles
B. the misfolded proteins are immediately sent to lysosomes
C. incompletely assembled proteins aggregate into large complexes that are degraded in the ER
D. misfolded proteins can exit the ER but are immediately returned by a KDEL sequence

Answer 1

Misfolded proteins are retained in the ER by chaperone proteins that assist in correct folding or, if unsuccessful, target them for degradation. Some can also be returned to the ER from the Golgi if they contain a KDEL sequence.

Step-by-step explanation:

Misfolded proteins and incompletely assembled proteins are retained in the endoplasmic reticulum (ER) primarily through the action of chaperone proteins. These chaperone proteins bind to the misfolded or incomplete proteins and assist in their correct folding or assembly. If the proteins cannot be correctly folded, they are targeted for degradation within the ER to prevent them from causing harm once they reach their final destinations. Misfolded proteins that escape the ER may be retrieved back to the ER if they contain a KDEL sequence, which serves as a retrieval signal for proteins mistakenly transported to the Golgi apparatus. This system ensures that only properly folded and assembled proteins are sorted to vesicles that bud off from the trans Golgi stacks and move to their final destinations, such as the plasma membrane or other organelles.