The alpha-2-macroglobulin receptor-associated protein (RAP) is a glycoprotein that binds to the alpha-2-macroglobulin receptor, as well as to other members of the low density lipoprotein receptor family . RAP acts to inhibit the binding of all know ligands for these receptors, and may prevent receptor aggregation and degradation in the endoplasmic reticulum, thereby acting as a molecular chaperone . RAP may be under the regulatory control of calmodulin, since it is able to bind calmodulin and be phosphorylated by calmodulin-dependent kinase II .
RAP is comprised of three domains. Both domains 1 and 3 are involved in binding to the alpha-2-macroglobulin receptor, while domain 1 is also involved in inhibiting the binding of activated alpha-2-macroglobulin . Structural studies have revealed the RAP domain 1 to be comprised of a partly opened bundle of three helices, the first one being shorter than the other two.