The structure of aggrecan and its turnover in cartilage.

Aggrecan is the major proteoglycan in cartilage. It has a multidomain structure with 3 globular and 2 extended segments. It forms large aggregates by binding to hyaluronan via the G1 domain, and link protein stabilizes the aggrecan-hyaluronan bond. The extended interglobular domain joining G1 and G2 domains is the main site of proteolytic attack in aggrecan turnover. One site of cleavage is reported to predominate, but the enzyme responsible for this cleavage has not been identified. A metalloproteinase, neutrophil collagenase, has been shown to cleave at this "aggrecanase" site in vitro; however, it has yet to be shown if metalloproteinases are responsible for this activity in cartilage.