Molecular chaperone for LDL receptor-related proteins that may regulate their ligand binding activity along the secretory pathway. Interacts with the LRP1/alpha-2-macroglobulin receptor heavy and light chains; the interaction is transient and coincides with a reduction of ligand binding by the receptor (PubMed:1400426). Interacts with LRP2/glycoprotein 330 (PubMed:1400426). Interacts with LRP1B; binding is followed by internalization and degradation. Interacts with LDLR (By similarity). Interacts with SORL1 (By similarity). Interacts with LRP1; this interaction is followed by rapid internalization (By similarity). May be associated with receptors at the cell surface. N-glycosylated. Belongs to the alpha-2-MRAP family. Was originally thought to be Heymann nephritis antigen gp330.