Functional duplication of ligand-binding domains within low-density lipoprotein receptor-related protein for interaction with receptor associated protein, α2-macroglobulin, factor IXa and factor VIII

The low-density lipoprotein receptor-related protein (LRP) binds a range of proteins including receptor associated protein (RAP), activated α2-macroglobulin (α2M*), factor IXa (FIXa), and factor VIII (FVIII) light chain. The binding is mediated by the complement-type repeats, which are clustered in four distinct regions within LRP. Cluster II of 8 repeats (CR3–10) and cluster IV of 11 repeats (CR21–31) have been implicated in ligand-binding. Previous studies have aimed to identify the cluster II repeats involved in binding α2M* and RAP. We now evaluated the binding to RAP, α2M*, FIXa and FVIII light chain of triplicate repeat-fragments of not only clusters II but also of cluster IV. Employing surface plasmon resonance analysis, we found that most efficient ligand-binding was displayed by the repeats within region CR4–8 of cluster II and within region CR24–28 of cluster IV. Whereas the binding to RAP could be attributed to two consecutive repeats (CR5–6, CR26–27), combinations of three repeats showed most efficient binding to FIXa (CR6–8, CR26–28), FVIII light chain (CR5–7, CR6–8, CR24–26), and α2M* (CR4–6, CR24–26). The results imply that there is an internal functional duplication of complement-type repeats within LRP resulting in two clusters that bind the same ligands.