HEPARIN BINDING PROTEIN-44 (HBP-44)/RECEPTOR-ASSOCIATED PROTEIN (RAP) MEDIATES CELL-SUBSTRATUM ADHESION OF MOUSE NIH/3T3 CELLS THROUGH ITS BINDING TO LOW DENSITY LIPOPROTEIN (LDL) RECEPTOR-RELATED PROTEIN (LRP)

The low density lipoprotein receptor-related protein (LRP) is a multifunctional endocytic receptor with the ability to bind and endocytose several structurally and functionally distinct ligands. The 39 kDa receptor-associated protein (RAP) is an endoplasmic reticulum (ER) resident protein, which is believed to function intracellularly as a molecular chaperone for LRP and to regulate its ligand binding activity along the secretory pathway. Mouse heparin binding protein-44 (HBP-44) is a homologue of human RAP. Using a recombinant form of HBP-44 expressed in Escherichia coli cells as a highly specific ligand for LRP, we demonstrated that HBP-44 coated on cell culture plates mediates the cell-substratum adhesion of mouse 3T3 fibroblasts in a dose-dependent manner, with 50% attachment at the concentration of 0.2 μg/ml. Ligand blot analysis with HBP-44 of whole cell extracts and the materials precipitated by anti-LRP antibodies revealed that the receptor for HBP-44 on NIH/3T3 cells was LRP. The results suggest ...