Proteins with the KDEL signal are retrieved from:
Lumenal residents of endoplasmic reticulum that perform folding, such as BiP/GRP78, protein disulfide isomerase, calreticulin and Grp94, must be retained despite continuous bulk flow to Golgi. Retention is achieved by C-terminal tetrapeptide KDEL or variants HDEL in yeast for soluble proteins and Lys-Lys-X-X or Arg-X-Arg motifs for type I membrane proteins. Escaped proteins reach cis-Golgi and ER-Golgi intermediate compartment where seven-transmembrane KDEL receptor Erd2, actually three isoforms KDELR1-3 in mammals, cycles constitutively between ER and Golgi. At mildly acidic Golgi pH near 6.2
Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 12: KDEL Retrieval via Erd2 and COPI Vesicles.