Hyaluronan and Proteoglycan Link Protein 4, (HAPLN4), also known as brain link protein 2, is one of four secreted HAPLN family proteoglycans that bind hyaluronan. HAPLN4 contains one Ig-type C2-set domain and two link domains. Predominantly expressed in brain, HAPLN4 participates in the development of the perineuronal matrix. Human and mouse HAPLN4 share 91% aa sequence identity.
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.