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/alpha2-macroglobulin receptor (LRP/alpha2MR) and inhibits the binding of ligands to this. receptor. We recently reported that inhib ition of tissue-type plasminogen activator binding to LRP/alpha2MR is mediated by both amino-terminal and carboxyl-terminal regions of the 3 9-kDa protein, whereas inhibition Of alpha2-macroglobulin-proteinase b inding is mediated only by amino-terminal regions. In this report we s how that amino-terminal and carboxyl-terminal regions of the 39-kDa pr otein bind specifically and with high affinity to LRP/alpha2MR on rat hepatoma MH1C1 cells. Following binding, these amino-terminal and carb oxyl-terminal regions of the 39-kDa protein are each rapidly endocytos ed and degraded with kinetics identical to the full-length 39-kDa prot ein. Competition binding experiments with these constructs demonstrate that amino-terminal and carboxyl-terminal regions of the 39-kDa prote in compete with one another for binding to LRP/alpha2MR. A model is pr oposed in which amino-terminal and carboxyl-terminal regions of the 39 -kDa protein bind to different sites on LRP/alpha2MR in order to inhib it ligand binding.