In the secretory pathway, vesicles transport proteins from one cellular compartment to another in forward transport to deliver the protein to its correct location. Occasionally, misfolded proteins and incorrect proteins escape their original compartments, and a retrieval pathway is used to return the escaped proteins to their original compartment.
The ER uses many checkpoints to prevent the entry of incorrectly folded or a resident protein as cargo onto a transport vesicle. These mechanisms include cargo selectivity of the receptors, aggregation of functionally similar proteins, and retention sequences on resident proteins.
Biochemical studies characterizing the cargo-sorting receptors have shown that they are ligand-specific and bind only at specific pH (pH 5.0 in yeast and mammals). Functionally similar ER-resident can proteins aggregate to form large complexes that cannot be loaded onto transport vesicles, thus avoiding the accidental escape of these proteins from the organelle. Additionally, the membrane-spanning domain of several ER and Golgi proteins contain a retention sequence that marks these proteins for staying back in the ER, so receptors do not bind them.
If an ER-resident protein escapes these checkpoints and gets loaded onto a Golgi-bound vesicle, a specific amino acid sequence called a retrieval signal helps the protein get recognized and transported back to the ER. The receptors involved in retrieval mechanisms recognize the retrieval signals at the carboxyl terminus of soluble proteins of the ER and in the cytoplasmic domain of some ER and Golgi membrane proteins. The receptors selectively capture such proteins and package them in vesicles that transport them back to the ER.
Two well-characterized retrieval signals are the carboxy-terminal tetrapeptide KDEL and KKXX found in many ER lumen resident proteins. BiP, a molecular chaperone, plays a vital role in preventing the aggregation of misfolded proteins. Interaction between BiP and unfolded proteins is mediated by a substrate-binding domain and a nucleotide-binding domain for ATPase activity. KDEL receptor in the post-ER compartments such as ERGIC and Golgi binds with the carboxyl-terminal retrieval signal of BiP and returns BiP to the ER via COPI vesicles.
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