KDEL motif interacts with a specific sequence in mammalian erd2 receptor

The ER retention of lumenal proteins is achieved by a process which involves binding of escaped proteins via the C-terminal KDEL-tags to a KDEL receptor (erd2 receptor) in a post-ER compartment and return of the protein-receptor complex back to the ER. The transmembrane topology of the human KDEL receptor, which is an integral membrane protein, has been proposed. We have synthesised sets of cellulose-bound overlapping peptides covering the complete se quence of the receptor to study the interaction of the erd2 receptor with lumenal ER proteins, CaBP1 and CaBP2. At the next stage, the proposed lumenal loops of the receptor were more closely mapped. A short sequence, essential for the protein binding to the most efficient binding site of the receptor, was identified as 22KIWK25, which is in accordance with one of the proposed structural models of the receptor. The binding was of high specificity and was almost completely inhibited by KDEL-containing soluble peptides. The phosphorylation state of CaBP1/CaBP2 did not affect their binding to the KDEL receptor.