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C-terminal receptor-associated protein (RAP), Domain 3 This subfamily is the C-terminal domain (D3) of receptor-associated protein, RAP, an antagonist and a specialized chaperone in the endoplasmic reticulum that binds tightly to members of the low-density lipoprotein (LDL) receptor family and prevents them from associating with other ligands. D3 is required for folding and trafficking of low-density lipoprotein receptor-related protein (LRP). In the mildly acidic pH of the Golgi, unfolding of RAP-D3 helical bundle facilitates dissociation of RAP from the LDL receptor type A (LA) repeats of LDLR family proteins. Also, RAP has 3 regions that interact weakly with heparin, two regions located in D3 and one in RAP domain 2 (D2). The double module of complement type repeats, CR56, of LRP binds many ligands including alpha2-macroglobulin, which promotes the catabolism of the Abeta-peptide implicated in Alzheimer's disease.
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