The receptor-associated protein (RAP) interacts with several resident proteins of the endoplasmic reticulum including a glycoprotein related to actin

The receptor-associated protein (RAP) is a chaperone found primarily in the endoplasmic reticulum (ER) that plays a necessary role in the folding and exocytic trafficking of members of the LDL receptor gene family including megalin and the LDL receptor-related protein (LRP). Recently, RAP has been shown to interact with a growing number of proteins including several that are unrelated to the LDL receptor family as well as new members of this rapidly expanding family. Based on these observations, we have applied chemical crosslinking procedures to identify additional novel RAP-interacting proteins, and thereby better characterize the scope of RAP's ER-related function. In this study, we have identified eight proteins with molecular weights of 32, 35, 46, 55, 70, 95, 170, and 200 kDa that interact with endogenous RAP. These proteins were found to associate with RAP in multiple cell types from different species, suggesting that their expression and interactions with RAP are ubiquitous. Results of pulse-chase experiments show that most of the proteins remain sensitive to endoglycosidase-H digestion, and also remain stably associated with RAP over an extended period, suggesting that they are ER resident proteins. All of the RAP-associated proteins appear to be largely soluble as they partition into the aqueous phase following TX-114 detergent extraction. Sequence analysis and immunoblotting of the 46-kDa RAP-associated glycoprotein (gp46) shows that it is structurally and immunologically related to actin. If gp46 is also functionally related to actin as an intracellular structural protein, it may represent a novel component of the putative ER matrix.