In the Golgi-ER retrograde pathway, sorting of membrane proteins occurs throughout the interaction of coat proteins with their cytoplasmic domains.
A putative dilysine motif is present within the C-terminal domain (-6 to -7) of the KDEL receptor.
This dilysine motif is required for interaction with coatomer (COPI is a coatomer, a protein complex that coats vesicles transporting proteins from the cis end of the Golgi complex back to the rough endoplasmic reticulum (ER), where they were originally synthesized, and between Golgi compartments).
It has been observed that if this dilysine motif is present but 3 amino acids from C-terminus are removed, KDEL receptor can not bind to coatomer. It means other factors are also responsible for binding. e.g. Phosphorylated Serine residue at 209 position is critical for binding.