[5][6][7] Retention of resident soluble proteins in the lumen of the endoplasmic reticulum (ER) is achieved in both yeast and animal cells by their continual retrieval from the cis-Golgi, or a pre-Golgi compartment.

Sorting of these proteins is dependent on a C-terminal tetrapeptide signal, lys-asp-glu-leu (KDEL) in animal cells and his-asp-glu-leu (HDEL) in S. cerevisiae.

This process is mediated by a receptor that recognizes, and binds the tetrapeptide-containing protein, and returns it to the ER.

In yeast, the sorting receptor encoded by a single gene, ERD2, is a seven-transmembrane protein.

KDELR2 was the second member of the family to be identified, and it encodes a protein which is 83% identical to the KDELR1 gene product.