CMP-NeuNAc:poly-α-2,8-sialosyl sialyltransferase and the biosynthesis of polysialosyl units in neural cell adhesion molecules

Prokaryotic derived probes that specifically recognize α-2,8-ketosidically linked polysialosyl units were developed to identify and study the temporal expression of these unique carbohydrate moieties in developing neural tissue. These polysialosyl units cap N-linked oligosaccharides of the complex-type on neural cell adhesion molecules (N-CAM). A Golgi-enriched fraction from 20-day-old fetal rat brain contains a membrane-associated sialyltransferase that catalyzes the incorporation of [¹⁴C]N-acetylneuraminic acid ([¹⁴C]NeuNAc) from CMP-[¹⁴C]NeuNAc into polymeric products. At pH 6.0, 84 pmol of NeuNAc mg of protein^-1 h^-1 were incorporated. In sodium dodecyl sulfate-polyacrylamide gels, the major radiolabeled species migrated with a mobility expected for N-CAM. A bacteriophage-derived endoneuraminidase specific for polysialic acid was used to demonstrate that at least 20-30% of the [¹⁴C]NeuNAc was incorporated into α-2,8-linked polysialosyl units. This was confirmed by structural studies which showed that the endoneuraminidase-sensitive brain material consisted of multimers of sialic acid. The addition of a partially purified preparation of chick N-CAM to the membranous sialyltransferase stimulated sialic acid incorporation 3-fold. The product of this reaction was also sensitive to endoneuraminidase and contained α-2,8-linked polysialosyl chains, thus showing that N-CAM can serve as an exogenous acceptor for sialylation in vitro. Sialic acid incorporated into adult rat brain membranes was resistant to endoneuraminidase, indicating that the poly-α-2,8-sialosyl sialyltransferase activity is restricted to an early developmental epoch. It is recommended that the enzyme described here be designated CMP-NeuNAc:poly-α-2,8-sialosyl sialyltransferase and the trivial name poly-α-2,8-sialosyl sialyltransferase be adopted.