Some proteins are so modified with CHOs that they contain more CHOs than amino acids. Proteins linked to glycosoaminoglycans are together called proteoglycans (PGs). The structure of a few proteoglycans is known. The GAGs are O-linked to the protein, typically to a Ser of a Ser-Gly dipeptide often repeated in the protein. Some of the proteoglycans also contain N-linked oligosaccharide groups.
PGs can be soluble and found in the extracellular matrix, or as integral membrane proteins. Given the diversity of sugars and the varying extent of sulfation, the CHO part of PGs provides an incredible variety of binding structures at or near to the cell surface. One PG, syndecan, binds through its intracellular domain to the internal cytoskeleton of the cell, while interacting with another protein - fibronectin - in the extracellular matrix. Fibronectin also binds other molecules which can regulate cellular growth and other interactions. PGs act like glue in connecting the extracellular and intracellular functions of the cell. Most proteins bind PGs through a PG binding motif of BBXB or BBBXXB where B is a basic amino acid. Some proteins bind to specific sequences in specific GAGs. For instance, antithrombin 3, an inhibitor of blood clotting, binds specifically to heparin, which enhances its interaction with the clotting protein thrombin.