Link proteins have a structure analogous to that of the G1 region of aggrecan, possessing A, B1 and B2 domains and are thus members of the hyalectan family, the family of proteoglycans to which aggrecan belongs (Neame and Barry, 1993). The A domain has been shown to be responsible for interaction with the G1 region of aggrecan, whereas the B domains interact with HA (Grover and Roughley, 1994). Intact human cartilage link protein can exist in two molecular forms (Fig. 3), termed LP1 and LP2, which differ by the presence of two or one N-linked oligosaccharide chains, respectively, in their N-terminal regions (Roughley et al., 1982). Proteolytic modification of the link proteins can occur in the region between the two oligosaccharide chains to yield a product termed LP3 (Mort et al., 1985). When the link proteins are present in a proteoglycan aggregate this site tends to be the only one that is accessible to most proteinases. Additional proteolysis does take place in vivo within the A domain, to produce LP fragments. This type of processing has been mimicked in vitro by the action of free radicals. Proteolytic modification of link protein takes place throughout life (Mort et al., 1983), with the modified link proteins accumulating in the proteoglycan aggregates. Matrix metalloproteinases have been implicated in such degradation (Nguyen et al., 1993), but the link proteins are resistant to aggrecanases (Sztrolovics et al., 1999). Mammals possess four link protein genes (Spicer et al., 2003), of which one is predominantly expressed in cartilage. Each LP gene is adjacent to a hyalectan gene, though it does not appear that adjacent gene products are necessarily co-expressed, as the cartilage LP gene is not adjacent to that of aggrecan. The cartilage LP serves several functions in the proteoglycan aggregate (Fig. 4). First by virtue of its ability to interact with both HA and the G1 domain of aggrecan it stabilizes the proteoglycan aggregate and prevents its dissociation under physiological conditions. Second, it participates in a phenomenon termed “delayed aggregation”, as newly secreted aggrecan has little ability to interact with HA and requires a LP-mediated conformational change in its G1 domain to promote aggregate formation (Melching and Roughley, 1990). Third, together with the G1 domain of aggrecan, LP forms a protein coat covering the surface of HA. This coat helps protect the HA from undesirable degradation by both hyaluronidases and free radicals.
The human cartilage link protein gene, which resides at chromosome 5q13-14 (Osborne-Lawrence et al., 1990), possesses 5 exons, with exon 3 encoding the A domain and exons 4 and 5 encoding the B domains (Dudhia et al., 1994). The importance of link protein in maintaining the structure of the proteoglycan aggregates has been demonstrated in knockout mice, which exhibit cartilage defects resulting in dwarfism and craniofacial abnormalities (Watanabe and Yamada, 1999). To date no human disorder has been directly linked to a mutation in link protein, but its importance in maintaining cartilage function is not in question.