Differential functions of triplicated repeats suggest two independent roles for the receptor-associated protein as a molecular chaperone

The 39-kDa receptor-associated protein (RAP) is a molecular chaperone for the low density lipoprotein receptor-related protein (LRP), a large endocytic receptor that binds multiple ligands. The primary function of RAP has been defined as promotion of the correct folding of LRP, and prevention of premature interaction of ligands with LRP within the early secretory pathway. Previous examination of the RAP sequence revealed an internal triplication. However, the functional implication of the triplicated repeats was unknown. In the current study using various RAP and LRP domain constructs, we found that the carboxyl-terminal repeat of RAP possesses high affinities to each of the three ligand-binding domains on LRP, whereas the amino-terminal and central repeats of RAP exhibit only low affinity to the second and the fourth ligand-binding domains of LRP, respectively. Using truncated soluble minireceptors of LRP, we identified five independent RAP- binding sites, two on each of the second and fourth, and one on the third ligand-binding domain of LRP. By coexpressing soluble LRP minireceptors and RAP repeat constructs, we found that only the carboxyl-terminal repeat of RAP was able to promote the folding and subsequent secretion of the soluble LRP minireceptors. However, when the ability of each RAP repeat to inhibit ligand interactions with LRP was examined, differential effects were observed for individual LRP ligands. Most striking, both the amino-terminal and central repeats, but not the carboxyl-terminal repeat, of RAP inhibited the interaction of α₂-macroglobulin with LRP. These differential functions of the RAP repeats suggest that the roles of RAP in the folding of LRP and in the prevention of premature interaction of ligand with the receptor are independent.