27.4: Fibril-associated Collagen

Fibril-associated collagens are a type of collagens present in the extracellular matrix with interrupted triple helices or FACIT (Fibril-associated collagens interrupted triple-helices). FACIT help connect and attach the collagen fibrils with each other as well as with other proteins of the extracellular matrix.

For example, the type II collagen fibrils in cartilage have covalently bound type IX fibril-associated collagens at regular intervals. Other types of fibril-associated collagens are XII, XIV, XVI, XIX, XX, XXI, and XXII. These collagens are mainly found in skin, tendon, articular cartilage, and cornea and provide structural support to the extracellular matrix of the connective tissue.

Fibril-associated collagens are site-specific and are distributed throughout the body according to their format of binding collagen fibrils. For example, type XII collagen is present in the human skeletal muscle. Type XIV collagen is usually found in the areas of high mechanical stress. Type IX collagen is found in the chondrocytes of cartilage and intervertebral discs. Type XVI collagen is present in dermal fibroblast, keratinocytes and smooth muscle cells. Type XXI collagen found in smooth muscle cells induces their proliferation and migration.

Fibril-associated collagens mediate the interactions of collagen fibrils with each other and with other components of the extracellular matrix.

For instance, Type VI fibril-associated collagens have a short triple helix with globular domains at both ends, which helps them associate with each other to form microfibrils.

Type VI collagen microfibrils are noncovalently bound to the surface of type I collagen fibrils present in tendons. This association forms stronger and thicker collagen fibers.

Similarly, type II collagen fibrils found in cartilage have covalently bound type IX fibril-associated collagens at regular intervals.

Type IX collagens have two to three triple-helical components linked by flexible kinks and a globular domain at the N-terminal, which protrudes out from the type II fibril.

Further, the flexible kink has a covalently bound chain of chondroitin sulfate, which also protrudes out from the fibril.

These protruding structures help connect type II fibrils to the components of the extracellular matrix.