Polysialic acid is an unusual posttranslational modification

Posttranslational modification

Posttranslational modification is the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis, and thus gene expression, for many proteins....

 that occurs on neural cell adhesion molecule

Neural Cell Adhesion Molecule

Neural Cell Adhesion Molecule is a homophilic binding glycoprotein expressed on the surface of neurons, glia, skeletal muscle and natural killer cells...

s (NCAM). Polysialic acid is considerably anionic. This strong negative charge gives this modification the ability to change the protein's surface charge and binding ability. In the synapse

Synapse

In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

, polysialation of NCAM prevents its ability to bind to NCAM's on the adjacent membrane.