Binding analysis of amino-terminal and carboxyl-terminal regions of the 39-kDa protein to the low density lipoprotein receptor-related protein

A 39-kDa protein binds to the low density lipoprotein receptor-related protein/α₂-macroglobulin receptor (LRP/α₂MR) and inhibits the binding of ligands to this receptor. We recently reported that inhibition of tissue- type plasminogen activator binding to LRP/α₂MR is mediated by both amino- terminal and carboxyl-terminal regions of the 39-kDa protein, whereas inhibition of α₂-macroglobulin-proteinase binding is mediated only by amino-terminal regions. In this report we show that amino-terminal and carboxyl-terminal regions of the 39-kDa protein bind specifically and with high affinity to LRP/α₂MR on rat hepatoma MH₁C₁ cells. Following binding, these amino-terminal and carboxyl-terminal regions of the 39-kDa protein are each rapidly endocytosed and degraded with kinetics identical to the full- length 39-kDa protein. Competition binding experiments with these constructs demonstrate that amino-terminal and carboxyl-terminal regions of the 39-kDa protein compete with one another for binding to LRP/α₂MR. A model is proposed in which amino-terminal and carboxyl-terminal regions of the 39-kDa protein bind to different sites on LRP/α₂MR in order to inhibit ligand binding.