An Evaluation of Sialation of the Nucleoporin p62

Many nuclear and cytosolic proteins are modified by single residues of O-linkedN-acetyl-d-glucosamine. These include many proteins found in nuclear pore complexes required for transport of macromolecules between the nucleus and the cytoplasm. The best characterized pore glycoprotein, p62, mediates its function as one component of a protein complex essential for nuclear transport. Although p62 sugar residues are not essential for nuclear transport, they appear to oppose protein phosphorylation occurring at sites predicted to destabilize protein–protein interactions of the p62 complex. Recently, a p62-like protein isolated from mouse neuroblastoma cells was reported to be modified by both GlcNAc and sialic acid. As there is little precedent for nucleoplasmic sialation, the finding that a characterized nuclear pore protein is sialated is significant because it may regulate pore function. To assess the biological importance of p62 sialation, GlcNAc and sialic acid-specific lectins were used to examine the state of p62 glycosylation in cells commonly used to study nuclear transport: frog eggs and normal rat kidney and HeLa fibroblasts. In addition, four mouse neuroblastoma cell lines derived from the same tumor were examined. The glycosylation of p62 in these cells appears to involve only single O-linked GlcNAc moieties; no significant sialation was detected.