Identification of protein disulfide isomerase 1 as a key isomerase for disulfide bond formation in apolipoprotein B100.
Bottom Line: Although these disulfide bonds were suggested to be important in maintaining apoB100 function, neither the specific oxidoreductase involved nor the direct role of these disulfide bonds in apoB100-lipidation is known.Here we used RNA knockdown to evaluate both MTP-dependent and -independent roles of PDI1 in apoB100 synthesis and lipidation in McA-RH7777 cells.Further, we demonstrate that PDI1 directly interacts with apoB100 via its redox-active CXXC motifs and assists in the oxidative folding of apoB100.
Affiliation: Degenerative Diseases Research Program, Sanford-Burnham Medical Research Institute, La Jolla, CA 92037.Show MeSH
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Mentions: To confirm that apoB100 is a substrate for PDI1, we analyzed the apoB100 protein complex in Pdi1-knockdown McA cells that stably express either HA-PDI1 or HA-PDI1CXXA as described before. Cell lysates were immunoprecipitated using apoB antibody, and the immunocomplexes were analyzed by SDS–PAGE under nonreducing conditions, followed by immunoblotting with anti-HA antibody. If apoB100 is a substrate of PDI1, the substrate-trap mutant of PDI1 should be present in covalent association with apoB100 under nonreducing conditions. Indeed, we detected HA-PDI1CXXA in the apoB100 immunocomplex under nonreducing conditions (Figure 5A), indicating that the cysteines of the CXXA motif in substrate-trap mutant PDI1 can form disulfide bonds with cysteine residues of apoB100. This observation indicates that PDI1 promotes formation of disulfide bonds in apoB100 via intermolecular disulfide bonds. Thus PDI1 is important for stabilizing apoB100 structure by serving as a thiol-disulfide oxidoreductase.
Affiliation: Degenerative Diseases Research Program, Sanford-Burnham Medical Research Institute, La Jolla, CA 92037.