Ligand binding properties of the very low density lipoprotein receptor - Absence of the third complement-type repeat encoded by exon 4 is associated with reduced binding of M-r 40,000 receptor-associated protein

The very low density lipoprotein receptor (VLDLR) binds, among other ligand s, the M-r 40,000 receptor-associated protein (RAP) and a variety of serine proteinase-serpin complexes, including complexes of the proteinase urokina se-type plasminogen activator (uPA) with the serpins plasminogen activator inhibitor-1 (PAI-1) and protease nexin-1 (PN-1). We have analyzed the bindi ng of RAP, uPA.PAI-1, and uPA.PN-1 to two naturally occurring VLDLR variant s, VLDLR-I, containing all eight complement-type repeats, and VLDLR-III, la cking the third complement-type repeat, encoded by exon 4. VLDLR-III displa yed similar to 4-fold lower binding of RAP than VLDLR-I and similar to 10-f old lower binding of the most C-terminal one of the three domains of RAP. I n contrast, the binding of uPA PAI-1 and uPA PN-1 to the two VLDLR variants was indistinguishable. Surprisingly, uPA PN-1, but not uPA PAI-1, competed RAP binding to both VLDLR variants. These observations show that the third complement-type repeat plays a crucial role in maintaining the contact sit es needed for optimal recognition of RAP, but does not affect the proteinas e-serpin complex contact sites, and that two ligands can show full cross-co mpetition without sharing the same contacts with the receptor. These result s elucidate the mechanisms of molecular recognition of ligands by receptors of the low density lipoprotein receptor family.