A Novel Mechanism for Controlling the Activity of cu2-Macroglobulin Receptor/Low Density Lipoprotein Receptor-related Protein

The az-macroglobulin receptor/low density lipopro- tein receptor-related protein (a2MR/LRP) consists of two polypeptides, 515 and 85 kDa, that are noncova- lently associated. A 39-kDa polypeptide, termed the receptor-associated protein (RAP), interacts with the 516-kDa subunit after biosynthesis of these molecules and remains associated on the cell surface. This mole- cule regulates ligand binding of a2MR/LRP (Herz, J., Goldstein, J. L., Strickland, D. K., Ho, Y. K., and Brown, M. S. (1991)