ERGIC-53 is a functional mannose-selective and calcium-dependent human homologue of leguminous lectins

Based on sequence homologies with leguminous lectins, the intermediate compartment marker ERGIC-53 was proposed to be a member of a putative new class of animal lectins associated with the secretory pathway. Independent, a promyelocytic protein, MR60, was purified by mannose-column chromatography, and a cDNA was isolated that matched MR60 peptide sequences. This cDNA was identical to that of ERGIC-53 and homologies with the animal lectin family of the galectins were noticed. Not all peptide sequences of MR60, however, were found in ERGIC-53, raising the possibility that another protein associated with ERGIC-53 may possess the lectin activity. Here, we provide the first direct evidence for a lectin function of ERGIC-53. Overexpressed ERGIC-53 binds to a mannose column in a calcium-dependent manner and also co-stains with mannosylated neoglycoprotein in a morphological binding assay. By using a sequential elution protocol we show that ERGIC-53 has selectivity for mannose and low affinity for glucose and GlcNAc, but no affinity for galactose. To experimentally address the putative homology of ERGIC-53 to leguminous lectins, a highly conserved protein family with an invariant asparagine essential for carbohydrate binding, we substituted the corresponding asparagine in ERGIC-53. This mutation, as well as a mutation affecting a second site in the putative carbohydrate recognition domain, abolished mannose-column binding and co-staining with mannosylated neoglycoprotein. These findings establish ERGIC-53 as a lectin and provide functional evidence for its relationship to leguminous lectins. Based on its monosaccharide specificity, domain organization, and recycling properties, we propose ERGIC-53 to function as a sorting receptor for glyco-proteins in the early secretory pathway.