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: Previous work suggested that the oxidoreductase activity of PDI1 is required for MTP-independent steps of apoB100 synthesis and secretion (Wang et al., 1997). To explore this possibility in more detail, we used pulse-chase experiments in which newly synthesized proteins were resolved by nonreducing gel electrophoresis to allow us to test whether Pdi1 knockdown modifies apoB disulfide bond formation. We found that intracellular and secreted apoB100 migrated more slowly at the later chase time points in Pdi1-knockdown cells, raising the possibility that disulfide bond formation is delayed for newly synthesized apoB100 (Figure 4A, asterisk). To our surprise, apoB100 with partially oxidized disulfide bonds were also secreted in Pdi1-knockdown cells, suggesting that these apoB100 molecules undergo sufficient lipidation to attenuate posttranslational degradation.
Affiliation: Degenerative Diseases Research Program, Sanford-Burnham Medical Research Institute, La Jolla, CA 92037.