Prominent 85-kDa oligomannosidic glycoproteins of rat brain are signal regulatory proteins and include the SHP substrate-1 for tyrosine phosphatases

The glycoprotein component in rat brain reacting most strongly with Galanth us nivalis agglutinin (GNA) on western blots migrates as an 85-kDa band. GN A identifies mannose-rich oligosaccharides because it is highly specific fo r terminal alpha-mannose residues. After purification of this 85-kDa glycop rotein band by chromatography on GNA-agarose and preparative gel etectropho resis, binding of other lectins demonstrated the presence of fucose and a t race of galactose, but no sialic acid. Treatment with N-Glycanase or endogl ycosidase H produced a 65-kDa band, indicating that it consisted of about o ne-fourth N-linked oligomannosidic carbohydrate moieties. High-performance anion-exchange chromatography and fluorescence-assisted carbohydrate electr ophoresis indicated that the major carbohydrate moiety is a heptasaccharide with the structure Man alpha 1-6(Man alpha 1-3)Man alpha 1-6(Man alpha 1-3 )Man beta 1-4Glc-NAc beta 1-4GlcNAc (Man(5)GlcNAc(2)). Determination of ami no acid sequences of peptides produced by endoproteinase digestion demonstr ated that this 85-kDa mannose-rich glycoprotein component contained the SHP substrate-1 for phosphotyrosine phosphatases and at least one other member of the signal-regulatory protein (SIRP) family. The unusually high content of oligomannosidic carbohydrate moieties on these receptor-like members of the immunoglobulin superfamily in neural tissue could be of functional sig nificance for intercellular adhesion or signaling.