NCAM in the developing brain is highly polysialylated and is referred as the embryonic form of NCAM. Polysialic acid is a developmentally regulated, anti-adhesive glycan with a linear homopolymer of alpha2,8-linked sialic acid units. They are mainly attached to the fifth and sixth N-glycosylation sites of the fifth Ig-like domain of NCAM. Polysialylation of NCAM is catalyzed by two polysialyltransferases, ST8Sia II (STX) and ST8Sia IV (PST), which belong to the family of six genes encoding alpha2,8-sialyltransferases. These enzymes add polysialic acid to NCAM N-glycans until it reaches a certain size (up to 200 sialic acid residues), where neither enzyme can interact with polysialylated N-glycans, and the polymerization of sialic acid is terminated.
Due to the structure with its chemical nature, polysialic acid can attenuate the interaction of NCAM with NCAM and other molecules in the same membrane (cis-interaction) or in another cell membrane (trans-interaction). During axonal growth the presence of polysialic acid along axons seems to prevent inappropriate synapse formation.