α-(2 → 8)-Polysialic acid immunoreactivity in voltage-sensitive sodium channel of eel electric organ

Abstract

The voltage-sensitive sodium channel from eel electroplax is formed of a polypeptide of 208321 Da, to which is attached ca . 85 kDa of carbohydrate. Sialic acid is a prominent constituent, contributing ca . 113 negative charges to the protein surface. We here demonstrate that antibodies raised against the bacterial antigen α-(2 → 8)-polysialic acid, specific for polymers of ten or more consecutive sialic acid residues, react specifically and with high affinity to the electroplax sodium channel. In extracts of electroplax membranes, the sodium channel is the only protein that demonstrates this immunoreactivity, suggesting the presence of a polysialosyl-sialyltransferase specifically committed to this unique post-translational modification of the sodium channel. Polysialic acid is rare in vertebrates, having previously been found only associated with neuralcell adhesion molecules, present in the developing neuromuscular system. The other prominent source is the capsular polysaccharide of highly pathogenic meningitis bacteria. Antibodies to the bacterial antigen thus provide highly specific affinity markers for the sodium channel. The high avidity of these antibodies and the ratio of sialic acid residues to consensus glycosylation sites suggest that the terminal chains are well over ten sialosyl residues in length, potentially extending 10–30 nm into the extracellular environment.