POLYSIALYLATION OF NCAM BY A SINGLE ENZYME

The addition of poly-alpha 2,8-N-acetylneuraminic acid (polysialic aci d; PSA) to the neural cell adhesion molecule NCAM plays a crucial role in neural development [1-3], neural regeneration [4], and plastic pro cesses in the vertebrate brain associated with neurite outgrowth [5], axonal pathfinding [6], and learning and memory [7-9]. PSA levels are decreased in people affected by schizophrenia [10], and PSA has been i dentified as a specific marker for some neuroendocrine and lymphoblast oid tumours [11-13]; expression of PSA on the surface of these tumour cells modulates their metastatic potential [11,13]. Studies aimed at u nderstanding PSA biosynthesis and the dynamics of its production have largely been promoted by the cloning of polysialyltransferases (PST-1 in hamster; PST in human and mouse) [14-16]. However, the number of en zymes involved in the biosynthesis of PSA has not been identified. Usi ng incompletely glycosylated NCAM variants and soluble recombinant gly cosyltransferases, we reconstituted the site at which PST-1 acts to po lysialylate NCAM in vitro. The data presented here clearly demonstrate that polysialylation of NCAM is catalyzed by a single enzyme, PST-1, and that terminal sialylation of the N glycan core is sufficient to ge nerate the PSA acceptor site. Our results also show that PST-1 can act on core structures with the terminal sialic acid connected to galacto se via an alpha 2,3 or alpha 2,6 linkage.