Abstract
The 39-kDa receptor-associated protein (RAP) is a specialized antagonist that inhibits all known ligand interactions with receptors that belong to the low density lipoprotein (LDL) receptor gene family. Recent studies have demonstrated a role for RAP as a molecular chaperone for the LDL receptor-related protein during receptor folding and trafficking within the early secretory pathway. In the present study, we investigated a potential role for RAP as a chaperone for the very low density lipoprotein (VLDL) receptor, another member of the LDL receptor gene family. Using intracellular cross-linking techniques, we found that RAP is associated with newly synthesized VLDL receptor. In the absence of RAP co-expression, newly synthesized VLDL receptor exhibited slower trafficking along the early secretory pathway, most likely due to misfolding of the receptor. The role of RAP in the folding of the VLDL receptor was further studied using an anchor-free, soluble VLDL receptor. Metabolic pulse-chase labeling experiments showed that while only 3% of the soluble VLDL receptor was folded and secreted in the absence of RAP co-expression, over 50% of the soluble receptor was secreted in the presence of RAP co-expression. The functions of RAP in VLDL receptor folding and trafficking were mediated by its carboxyl-terminal repeat but not by the amino-terminal and central repeats. Using truncated VLDL receptor constructs, we identified the RAP-binding site within the first three ligand-binding repeats of the VLDL receptor. Thus, our present study demonstrates that RAP serves as a folding and trafficking chaperone for the VLDL receptor via interactions of its carboxyl-terminal repeat with the three amino-terminal ligand-binding repeats of the VLDL receptor.
Highlights
§ Supported in part by a grant from STINT, The Swedish Institute, and by a fellowship from SANOFI Association for Thrombosis Research
receptor-associated protein (RAP) Is Associated with the very low density lipoprotein (VLDL) Receptor in Vivo—To test whether RAP is associated with the VLDL receptor intracellularly, we examined if RAP could be cross-linked to the VLDL receptor in intact cells
Since RAP functions as an antagonist of ligand binding to other members of the low density lipoprotein (LDL) receptor gene family, it was hypothesized that RAP serves as a molecular chaperone for these other receptors
Summary
§ Supported in part by a grant from STINT, The Swedish Institute, and by a fellowship from SANOFI Association for Thrombosis Research. In the absence of RAP co-expression, high molecular weight aggregates of misfolded LRP are seen when analyzed under nonreducing SDS-PAGE. These aggregates are probably due to the formation of intermolecular disulfide bonds during misfolding, tein receptor-related protein; MEF, mouse embryonic fibroblast; PAGE, polyacrylamide gel electrophoresis; RAP, receptor-associated protein; VLDL, very low density lipoprotein; Endo H, endoglucosidase H; HA, hemagglutinin. The objective of the current study was to investigate whether RAP serves as a molecular chaperone during the folding and trafficking of the VLDL receptor, and if so to identify the structural basis for RAP-VLDL receptor interaction. We found that interaction between the carboxyl-terminal repeat of RAP and the three amino-terminal repeats of the VDLD receptor plays important role in the folding and trafficking of this receptor
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