KDEL sequence and retrieval of ER-resident proteins
KDEL, a C-terminal tag (at the end of the chain made last), lets a Golgi receptor return escaped soluble proteins, which are not anchored in place, to the endoplasmic reticulum.
Remember
A C-terminal KDEL tag marks soluble ER proteins for return from the Golgi: the receptor grips it in the acidic pH there and lets go at the neutral pH of the ER.
Memory aid
Think of KDEL as a return-to-sender label
If a protein wanders to the Golgi, the label sends it home.
Many proteins that work inside the endoplasmic reticulum (ER, the membrane network where proteins are folded and processed), such as chaperones that help other proteins fold, are soluble, which means they are not anchored in place. Vesicles (small membrane packages) carry material towards the Golgi apparatus, so these proteins can be swept along with it.
They are rescued by a short tag of four amino acids at the C-terminus: lysine, aspartate, glutamate and leucine, written KDEL.
A KDEL receptor in the cis-Golgi (the face of the Golgi nearest the ER) binds the tag, and the acidic pH there strengthens the binding. The receptor and its cargo (the escaped protein) are carried back to the ER by retrograde transport, which means transport backwards. In the ER the neutral pH makes the protein let go, and the receptor returns to the Golgi for another round.
A simple memory aid is that acid grips and neutral lets go. If the cis-Golgi became near-neutral, the receptor would bind the tag less well, retrieval would fall, and more of these proteins would move on along the secretory pathway (the route taken by proteins bound for secretion) instead of returning.
Key facts
- Tag
- a C-terminal Lys-Asp-Glu-Leu (KDEL) sequence marks soluble ER proteins for retrieval.
- Receptor
- a KDEL receptor in the cis-Golgi binds the tag.
- pH dependence
- binding is enhanced by the acidic pH of the Golgi and lost at the neutral pH of the ER.
- Route back
- the receptor and the protein return to the ER by retrograde transport, and the receptor then goes back to the Golgi for another round.
- Loss of acidity
- a near-neutral Golgi would weaken binding, so retrieval would fall and more soluble ER proteins would move on along the secretory pathway.
- Untagged soluble proteins
- those that reach the Golgi are not retrieved and move on along the secretory pathway.
Exam traps
- A signal peptide (the entry signal that first sends a growing polypeptide into the ER) does not keep a protein in the ER, because it is removed inside the ER, so a protein with only this signal has no tag to bring it back from the Golgi.
- A C-terminal serine-lysine-leucine tag looks like KDEL but directs proteins to peroxisomes (small organelles that handle lipid metabolism and detoxification), not back to the ER.
- KDEL binding is not stronger at neutral pH: it is strongest in the acidic Golgi, so raising the Golgi pH weakens binding and reduces retrieval.
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