The transmembrane KDEL receptor is abundant in the Golgi, but does not return to

Question

The transmembrane KDEL receptor is abundant in the Golgi, but does not return to the ER unless it has bound a soluble ER protein Why do you think the KDEL receptor only returns to the ER after it has bound an ER protein in the Golgi? A The KDEL receptor cannot bind GTP unless it has also bound an ER protein The KDEL receptor does not have a binding site for the COPI coat proteins, so it can only return when the aqueous-soluble ER protein supplies the binding site The Rab GTPase will reject any vesicle containing KDEL receptors without bound ER proteins The coat-recruitment GTPase blocks the KDEL receptor from entering the COPI vesicle unless it has bound an ER protein Binding the ER protein causes a change in conformation in the KDEL receptor that exposes a binding site for the COPI coat adaptor protein

Explanation / Answer

B. and E both are correct answers.

The KDEL receptor does not have binding site for COPI protein; and without COPI protein retrograde transport cannot occur. This binding to COPI protein will occur only when KDEL receptor will bind to ER protein. Remember that KDEL receptor has very high affinity for ER proteins which have moved in the Golgi.

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