Proteoglycans are most abundant in the cartilage of vertebrates. Individual proteoglycans and hylauronic acid proteoglycan complexes will directly like to collagen fibers, which forms the fiber-network complex on animal extracellular structures. Tendons, which are almost completely made of pure collagen fibers, are tough and elastic, as compared to the soft cartilage, which contains proteoglucans in high proportions. In bones, the combination of collagen fibers, proteoglycans, and minerals produces a structure that is extremely dense, hard, and has both elasticity and a high tensile strength. Proteoglycans also function as sites for cell adhesions, both temporary and permanent.
Some cells link to the collagen-proteoglycan complex directly.
This may be set up through the surface receptors which are embedded into the plasma membrane. These receptors recognize and bind collagen or proteoglycans in the membrane. Direct linkage of cell surfaces to collagens is reinforced by the attachments that are made through one or more of a group of linker molecules , which make noncovalent bonds with both the collagen and proteoglucan molecules, along with the receptors. The two most familiar linker molecules are fibronectins , which occur in all extracellular surfaces, and laminin , which is restricted to basal laminae. Fibronectins are large glycoproteins which are assembled from two fibrous polypeptides that are very similarly structured.