ERGIC-53, A MEMBRANE-PROTEIN OF THE ENDOPLASMIC RETICULUM-GOLGI INTERMEDIATE COMPARTMENT, IS IDENTICAL TO MR60, AN INTRACELLULAR MANNOSE-SPECIFIC LECTIN OF MYELOMONOCYTIC CELLS

A mannose specific membrane lectin (MR60) isolated from human myelomon ocytic HL60 cells by affinity chromatography is expressed in intracell ular organelles of immature monocytes (Pimpaneau, V., Midoux, P., Mons igny, M., and Roche, A. C. (1991) Carbohydr. Res. 213, 95-108). It is not present at the cell surface and is immunochemically and structural ly distinct from the M(r) 175,000 mannose receptor of mature macrophag es. MR60 cDNA was isolated and characterized; on the basis of its sequ ence, MR60 is not related to any known mammalian lectins. Surprisingly , MR60 was found to be identical to ERGIC-53 (Schindler, R., Itin, C., Zerial, M., Lottspeich, F., and Hauri, H. P. (1993) Eur. J. Cell Biol . 61, 1-9), a type I integral membrane protein, defined as a marker of the intermediate compartment that recycles between the Golgi apparatu s and endoplasmic reticulum; MR60/ERGIC-53 shares with VIP-36 signific ant homologies with leguminous plant lectins (Fiedler, K., and Simmons , R. (1994) Cell 77, 625-626). We extend these findings in evidencing a structural homology between MR60/ERGIC-53 and mammalian galectins (s oluble beta galactose binding proteins). MR60/ERGIC-53 is the first le ctin characterized as an endoplasmic reticulum-Golgi protein. Accordin gly, this intracellular mannose binding protein could be involved in t he traffic of glycoproteins between endoplasmic reticulum and the Golg i apparatus.