Forssman disaccharide is the specific ligand of a galectin from the sponge Geodia cydonium but does not mediate its binding to nuclear protein np56.

The galectin from Geodia cydonium (GCA) had previously been shown to be involved in regulatory mechanisms of cell sorting and adhesion during reaggregation of allogeneic sponge cells. In this contribution the binding specificity of GCA was established to be GalNAc alpha 1-3GalNAc beta as structural component of Forssman pentasaccharide. Crossreactivities of terminal structural elements were revealed in the order GalNAc alpha 1-3GalNAc beta > GalNAc alpha 1-3(Fuc alpha 1-2)Gal beta >> Gal beta 1-3GlcNAc beta > Gal beta 1-4Glc. Lectin binding to the Forssman antigen (Ki range 10(-7)) or to blood group A-trisaccharide exceeded that to lactose (Ki range 10(-3)/10(-2)) by three to four orders of magnitude. Cytochemical staining of eukaryotic cells on the light and electron microscopic level revealed lectin binding in the cytosol and in the nucleus (nucleoli), which was inhibitable with the soluble high affinity ligands. The nuclear binding of GCA could be ascribed to affinity-isolated 56 kDa protein (np56) in the nucleoplasm and was shown to be mediated by the peptide conformation of the ligand. Although GCA-np56 interaction was inhibitable with Forssman glycolipid or globopentaose, the carbohydrate binding site of the lectin is not involved due to the lack of competition by Forssman-specific lectins HPA or DBA. Since anti-CBP70 was immunologically cross-reactive to np56, it is concluded that the galectin GCA binds to np56 via similar mechanisms as reported previously for the interaction of CBP-35 (galectin-3) and CBP-70. Thus, GCA resembles galectin-3 in its binding characteristics but is likewise related to galectin-1 by sequence homology of its primary structure and by the molecular mass of its subunits.