The low-density lipoprotein receptor-related protein associates with calnexin, calreticulin, and protein disulfide isomerase in receptor-associated-protein-deficient fibroblasts

Abstract

The low-density lipoprotein receptor-related protein (LRP) is a large (>600 kDa) multi-ligand-binding cell surface receptor that is now known to participate in a diverse range of cellular events. To accomplish this diverse role, LRP is composed of repetitive amino acid motifs consisting of complement-type and EGF precursor-type repeats. Within these repeats are six conserved cysteine residues that form the core disulfide bond structure of each repeat. To accommodate the intricate folding that such a complex structure dictates, a specialized chaperone is present in the endoplasmic reticulum (ER) called the receptor-associated protein (RAP) that binds to LRP immediately following its biosynthesis and assists in its exocytic transport. Interestingly, RAP −/− mice show reduced LRP expression in certain cell types, but not a more global affect on LRP expression that was expected. Such a tissue-restricted effect by RAP prompted an investigation if other ER chaperones associate with LRP to assist in its complex folding requirements and compensate for the absence of RAP in RAP −/− cells. Fibroblasts obtained from RAP −/− mice demonstrate similar LRP expression levels and subcellular distribution as RAP +/+ fibroblasts. Moreover, RAP −/− cells show an identical exocytic trafficking rate for LRP as RAP +/+ cells and comparable cell surface internalization kinetics. In RAP −/− cells, three well-known ER chaperones, calnexin, calreticulin, and protein disulfide isomerase (PDI), associate with LRP and likely compensate for the absence of RAP.