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The link domain is a hyaluronan (HA)-binding domain. It functions to mediate adhesive interactions during inflammatory leukocyte homing and tumor metastasis. It is found in the CD44 receptor and in human TSG-6. TSG-6 is the protein product of the tumor necrosis factor-stimulated gene-6. TSG-6 has a strong anti-inflammatory effect in models of acute inflammation and autoimmune arthritis and plays an essential role in female fertility. This group also contains the link domains of the chondroitin sulfate proteoglycan core proteins (CSPG) including aggrecan, versican, neurocan, and brevican and the link domains of the vertebrate HAPLN (HA and proteoglycan binding link) protein family. In cartilage, aggrecan forms cartilage link protein stabilized aggregates with HA. These aggregates contribute to the tissue's load bearing properties. Aggregates in which other CSPGs substitute for aggregan might contribute to the structural integrity of many different tissues. Members of the vertebrate HPLN gene family are physically linked adjacent to CSPG genes. TSG-6 contains a single link module which supports high affinity binding with HA. The functional HA-binding domain of CD44 is an extended domain comprised of a link module flanked with N-and C- extensions. These extensions are essential for folding and functional activity. CSPGs are characterized by an N-terminal globular domain (G1 domain) containing two contiguous link modules (modules 1 and 2). Both link modules of the G1 domain of the CSPG aggrecan are involved in interaction with HA. Aggrecan in addition contains a second globular domain (G2) which contains link modules 3 and 4 which lack HA-binding activity. HAPLNs contain two contiguous link modules.
Jiyao W et al.(2023). "The conserved domain database in 2023.",