THE LINK PROTEINS

Aggregates of chondroitin-keratan sulfate proteoglycan (aggrecan) and hyaluronic acid (hyaluronan) are the major space-filling components of cartilage. A glycoprotein, link protein (LP; 40-48 kDa) stabilizes th e aggregate by binding to both hyaluronic acid and aggrecan. In the ab sence of LP, aggregates are smaller (as estimated by rotary shadowing of electron micrographs) and less stable (they dissociate at pH 5) tha n they are in the presence of LP. The proteoglycan aggregate, includin g LP, is dissociated in the presence of chaotropes such as 4 M guanidi ne hydrochloride. On removal of the chaotrope, the complex will reasso ciate. This forms the basis of the isolation of LP from cartilage and has been described in detail elsewhere. Tryptic digestion of the prote oglycan aggregates results in a high molecular weight product that con sists of hyaluronic acid to which is bound LP and the N-terminal globu lar domain of aggrecan (hyaluronic acid binding region; HABR) in a 1:1 stoichiometry. The amino acid sequences of LP and HABR are surprising ly similar. The amino acid sequence can be divided into three domains; an N-terminal domain that falls into the immunoglobulin super-family and two C-terminal domains that are similar to each other. The DNA str ucture echoes this similarity, in that the major domains are reflected in three separate exons in both LP and HABR. The two C-terminal domai ns are largely responsible for the association with HA and are related to two recently described hyaluronate-binding proteins, CD44 and TSG- 6. A variety of approaches, including analysis of the forms of LP foun d in vivo, rotary shadowing and analysis of the sequence in the immuno globulin-like domain, have shed considerable light on the structure-fu nction relationships of LP. This review describes the structure and fu nction of LP in detail, focusing on what can be inferred from the simi larity of LP, HABR and related molecules such as immunoglobulins and l ymphocyte HA-receptors.