Polysilic acid is an extended polymer of a-2,8-linked neuraminic acid found on the surfaces of neuroinvasive, pathogenic bacteria and on mammalian cells. As a posttranslational modification of the neuronal cell adhesion molecule (NCAM) polySia plays an important role in cellular motility, neuronal plasticity and has implications in tumor metastasis in mammals.

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A) Two endoNF monomers are displayed and coloured according to B-factor. In the close-up view the anisotropic displacement of surface residues is shown with 50% probability
B) Crystal structure of homotrimeric endoNF; the monomers are indicated by different shades of blue and green. The protein consists of an N-terminal linker domain, which is formed by the only a-helix of the structure, the catalytic sialidase â-propeller domain, into which the â-barrel domain is inserted and a stalk domain that is built by the intertwining of all three domains into a compact triple-â-helix. Sialic acid is shown as magenta spheres at the stalk-binding site. In the close-up view the interaction with the stalk domain is illustrated, which displays the electron density contoured at one sigma. Orange spheres display water molecules.