A regulatory circuit operated by KDELR1 and KDELR3 fine-tunes the composition of the early secretory pathway.

KDEL receptors (KDELRs) prevent the secretion of soluble chaperones and enzymes meant to reside in the endoplasmic reticulum. While a single KDELR exists in yeast (ERD2), three variants are present in mammals, displaying high sequence similarity. All three can prevent the secretion of KDEL-bearing clients. However, their diverse tissue distribution and the high phylogenetic conservation of the differences suggest functional specialization. Accordingly, we show here that while KDELR2 plays a major role in client retrieval, KDELR1 and KDELR3 regulate the production of AGR2, a key assistant of mucin folding, in opposite ways. AGR2 transcripts increase dramatically upon silencing KDELR3 but decrease when KDELR1 is downregulated. Silencing ERp44, but no other ER residents, phenocopies KDELR3 knockdown, suggesting that AGR2 regulation depends on ERp44-KDELR3 interactions. Our findings identify a novel regulatory circuit, distinct from the unfolded protein response, that controls the molecular composition of the early secretory pathway based on specific interactions between KDELRs and ER residents.