Receptor-associated protein binding blocks ubiquitinylation of the low density lipoprotein receptor-related protein

The low density lipoprotein receptor-related protein (LRP) consists of two subunits, M-r similar to 515,000 and 85,000. LRP is a receptor for activate d alpha (2)-macrogobulin (alpha M-2*), Pseudomonas exotoxin A, and many oth er proteins. We now report that ubiquitinylation of the LRP heavy chain occ urred when either Pseudomonas exotoxin A or alpha M-2* bound to LRP on macr ophages. Ubiquitinylation was dose-dependent and maximal about 30 min after ligation of the receptor. Addition of the proteosome inhibitor MG-132 sust ained the level of ubiquitin-LRP for longer time intervals in macrophages t reated with either alpha M-2* or Pseudomonas exotoxin A. By contrast, when receptor associated protein (RAP) bound to LRP, ubiquitinylation did not oc cur. While RA-P is not found in the extracellular environment it binds to L RP and is believed to function as an intracellular chaperone. The presence of RAP within the cell may, therefore, contribute to the recycling of intac t LRP which has ligated and internalized its ligands. (C) 2001 Elsevier Sci ence.