Mistletoe lectin I is a sialic acid-specific lectin with strict preference to gangliosides and glycoproteins with terminal Neu5Ac alpha 2-6Gal beta 1-4GlcNAc residues.

Mistletoe lectin I (ML-I) is a type II ribosome-inactivating protein, which inhibits the protein biosynthesis at the ribosomal level. ML-I is composed of a catalytically active A-chain with rRNA N-glycosidase activity anda B-chain with carbohydrate binding specificities. Using comparative solid-phase binding assays along with electrospray ionization tandem mass spectrometry, ML-I was shown to preferentially bind to terminally a2-6-sialylated neolacto series gangliosides from human granulocytes. IV 6 Neu5Ac-nLc4Cer, VI 6 Neu5Ac-nLc6Cer, and VIII 6 Neu5Ac-nLc8Cer were identified as ML-I receptors, whereas the isomeric α2-3-sialylated neolacto series gangliosides were not recognized. Only marginal binding of ML-I to terminal galactose residues of neutral glycosphingolipids with a Galβ1-4Glc or Galβ1-4GlcNAc sequence was determined, whereas a distal Galal-4Gal, GalNAcβ1 - 3Gal, or GalNAcβ1 -4Gal disaccharide did not bind at all. Among the glycoproteins investigated in Western blot and microwell adsorption assays, only those carrying Neu5Acα2-6Galβ1-4GlcNAc residues, exclusively, predominantly, or even as less abundant constituents in an assembly with Neu5Aca2-3Galβ1-4GlcNAc-terminated glycans, displayed high ML-I binding capacity. From our data we conclude that (i) ML-I has to be considered as a sialic acid- and not a galactose-specific lectin and (ii) neolacto series gangliosides and sialoglycoproteins with type II glycans, which share the Neu5Aca2-6Galβ1-4GlcNAc terminus, are true ML-I receptors. This strict preference might help to explain the immunostimulatory potential of ML-I toward certain leukocyte subpopulations and its therapeutic success as a cytotoxic anticancer drug.