Figure 1.

Domain structure of aggrecan and link protein. Aggrecan binds hyaluronan through its N-terminal G1 domain in ternary complex with link proteins. The G1 domain and link proteins are homologous, with an immunoglobulin-like repeat (A) followed by two proteoglycan tandem repeats (B and B′). In aggrecan, but not in other lecticans, the G1 is followed by a second globular domain (G2), separated from the G1 by an extended interglobular domain. The G2 domain consists of two proteoglycan tandem repeats (B and B′). The central, and largest, part of aggrecan is the glycosaminoglycan attachment region, an extended protein stretch carrying keratan sulfate and chondroitin sulfate chains. This is followed by the C-terminal G3 domain, which consists of an epidermal growth factor (EGF) repeat (E1), a calcium-binding EGF repeat (E2), a C-type lectin domain (L), and a complement regulatory protein repeat (C). Hyaluronan (HA) is shown in red, aggrecan in blue (core protein) with red glycosaminoglycan chains, link protein in green. The structural repeat composition of each globular domain of link protein and aggrecan is shown above and below the two proteins, respectively, as indicated by dashed lines.