ER Retrieval Pathway

ER retrieval signals are short amino acid sequences at the C-terminus of ER resident proteins that mark them for return if they escape the organelle. The KDEL sequence (lysine, aspartate, glutamate, leucine) and KKXX sequence (two lysines followed by two other amino acids) are the best-characterized retrieval signals. Receptors in the vesicular tubular clusters and Golgi recognize these sequences and capture escaped proteins for transport back to the ER via COPI vesicles.

BiP, a molecular chaperone, contains a KDEL retrieval signal at its C-terminus. When BiP is secreted from the ER and reaches post-ER compartments like the Golgi, the KDEL receptor recognizes and binds this sequence. The receptor packages BiP into COPI vesicles that transport it back to the ER. If the KDEL sequence is removed genetically, BiP is slowly secreted without retrieval.

The ER uses multiple checkpoints to retain resident proteins: cargo-selective receptors that bind only specific ligands at specific pH levels, aggregation of functionally similar proteins into large complexes too large for vesicles, and retention sequences in membrane-spanning domains. These mechanisms work together to prevent accidental loading of resident proteins onto Golgi-bound transport vesicles.

Functionally similar ER-resident proteins aggregate to form large complexes that cannot be loaded onto transport vesicles. This aggregation strategy prevents accidental escape of these proteins from the organelle during forward transport. By remaining as large complexes, resident proteins are physically excluded from being packaged into vesicles destined for the Golgi.

BiP is a molecular chaperone that prevents aggregation of misfolded proteins in the ER lumen. It interacts with unfolded proteins through a substrate-binding domain and uses ATPase activity via its nucleotide-binding domain to facilitate protein folding. BiP itself contains a KDEL retrieval signal, ensuring it remains in the ER to continue its protective function.

Cargo-selective receptors are ligand-specific and bind only at specific pH conditions, typically pH 5.0 in yeast and mammals. These receptors recognize retrieval signals at the carboxyl terminus of soluble ER proteins and in the cytoplasmic domains of some ER and Golgi membrane proteins. This specificity ensures that only escaped resident proteins are captured and packaged into retrieval vesicles.

When the KDEL retrieval signal is removed through genetic engineering, the protein loses its recognition sequence and is no longer captured by KDEL receptors. Without this signal, the protein is slowly secreted out of the ER without being returned. This demonstrates that the KDEL sequence is essential for the retrieval mechanism and ER retention of proteins like BiP.