5.

HSPG classifications and HS structure. (A) Heparan sulfate proteoglycans can be broadly classified as glypicans, perlecans and syndecans, depending on their distribution within and around the cell. Glypicans are attached to the cell membrane by GPI anchors, whereas syndecans are transmembrane proteins. Perlecans are actively secreted into the pericellular matrix and are predominantly found in the basement membrane. (B) Heparan sulfate is composed of repeating disaccharide units of glucuronic acid and glucosamine of varying length that can be modified at irregular intervals along the chain. These modifications cluster the sulfation patterns on the HS chain into discrete protein-binding domains and ultimately lead to different growth factor-binding specificities of different HS species. Copyright permission granted from Nature.