Glycosylation of mammalian neurofilaments. Localization of multiple O-linked N-acetylglucosamine moieties on neurofilament polypeptides L and M

Neurofilaments are neuronal intermediate filaments that play an important role in the growth and maintenance of large myelinated axons. Mammalian neurofilaments are composed of three polypeptide subunits, designed as NF-L, NF-M, and NF-H, all of which are phosphorylated. Here, we demonstrate by several criteria that neurofilament polypeptides are also modified by an abundant type of intracellular protein glycosylation in which single N-acetylglucosamine monosaccharides are O-glycosidically (O-GlcNAc) linked to serine or threonine residues. In purified neurofilament proteins, the O-GlcNAc modifications occur at a stoichiometry of approximately 0.1 and 0.15 mol of GlcNAc/mol of NF-L and NF-M, respectively. The predominant sites of O-GlcNAc attachment on NF-L and NF-M are identified using proteolysis, purification of the glycopeptides, and subsequent analysis by automated gas-phase sequencing, manual Edman degradation, and laser desorption mass spectrometry. For NF-L, both major sites of glycosylation (Thr21 and Ser27) are located at the NH2-terminal head domain. For NF-M, one major site (Thr48) lies within the NH2-terminal head domain, whereas the other (Thr431) is located at the tail domain. Deletions encompassing these sites have been shown previously to have a dominant detrimental effect upon neurofilament assembly, raising questions about the specific function(s) of the saccharide moieties at these sites. Specific identification of these O-GlcNAc attachment sites has set the stage for more detailed mutagenic analysis of O-GlcNAc functions on neurofilaments.