HAPLN1 (also known as link protein and CRTL1) is a member of the hyaladherin family of hyaluronic acid (HA) binding proteins. Hyaluronan binding proteins are of two types; those with link modules, and those without. Link modules are 100 amino acid (aa) HA and protein-binding sequences that contain two alpha-helices and two anti-parallel beta-sheets. There are three categories of link module-containing proteins. 'A' domain-type proteins contain one link module; 'B' domain-type proteins contain one link module with an N- and C-terminal flanking region; and 'C' domain-type proteins have an extended structure with one N-terminal V-type Ig-like domain followed by two link modules.
Mature human HAPLN1 is 97%, 96%, 96%, 96%, and 96% aa identical to equine, porcine, rat, mouse and bovine HAPLN1, respectively. HAPLN1 contributes to extracellular matrix stability and flexibility. In cartilage, HALPN1 forms a ternary complex with HA and aggrecan. This creates a gel-like substance with remarkable resistance to deformation. In this complex, HA forms a linear backbone with perpendicularly attached aggrecan and HAPLN1. Aggrecan and HAPLN1 lie parallel to each other, while HA runs between the two HAPLN1 link modules. The Ig domain of HAPLN1 binds to aggrecan, while the two link modules of HAPLN1 bind to HA. Although HA and aggrecan will associate, the tendency is towards dissociation. HAPLN1 provides a stabilizing influence on HA-aggrecan associations, thus creating a long-lived ternary functional complex.