Calreticulin (CRT) is well known as a lectin-like chaperone that recognizes Glc1Man9GlcNAc2 (G1M9)-glycoproteins in the endoplasmic reticulum (ER). However, whether CRT can directly interact with the aglycone moiety (protein portion) of the glycoprotein remains controversial. To improve our understanding of CRT interactions, structure-defined G1M9-derivatives with different aglycones (-OH, -Gly-NH2, and Gly-Glu-Bu-t) were used as CRT ligands, and their interactions with recombinant CRT were analyzed using thermal shift analysis. The results showed that CRT binds strongly to a G1M9-ligand in the order Gly-Glu-Bu-t > -Gly-NH2 > -OH, which is the same as that of the reglucosylation of Man9GlcNAc2 (M9)-derivatives by the folding sensor enzyme UGGT (UDP-glucose: glycoprotein glucosyltransferase). Our results indicate that, similar to UGGT, CRT discriminates the proximal region at the N-glycosylation site, suggesting a similar mechanism mediating the recognition of aglycone moieties in the ER glycoprotein quality control system. (C) 2015 Elsevier Inc. All rights reserved.