Alpha-2-macroglobulin receptor-associated protein, also known as low density lipoprotein receptor-related protein-associated protein 1, RAP and LRPAP1, is a 39 kDa protein and a member of the alpha-2-MRAP family. LRPAP1 is a receptor antagonist that interacts with several members of the low density lipoprotein (LDL) receptor gene family. Upon binding to these receptors, LRPAP1 inhibits all ligand interactions with the receptors. LRPAP1 is present on cell surface forming a complex with the alpha-2-macroglobulin receptor heavy and light chains.LRPAP1 is present on the cell surface, where it is an effective inhibitor of ligand binding to the kidney glycoprotein 330 (gp330) yet it is most abundant in the endoplasmic reticulum lumen and may function in receptor folding and/or trafficking.