![]() Under anaerobic conditions, the DsbB–DsbA system can support disulfide formation via alternate electron acceptors, such as fumarate ( Bader et al., 1999). Molecular oxygen can serve as the terminal electron acceptor for disulfide formation in both prokaryotes and eukaryotes ( Bader et al., 1999 Tu and Weissman, 2002). Both Ero1p and DsbB specifically oxidize a thioredoxin-like protein (PDI in eukaryotes, DsbA in bacteria) that serves as an intermediary in the transfer of oxidizing equivalents to folding proteins ( Bardwell et al., 1993 Frand and Kaiser, 1999 Tu et al., 2000). The conserved, ER-resident protein Ero1p plays an analogous role to the bacterial periplasmic protein DsbB in oxidative folding. However, the physiological relevance of these to oxidative folding has been unclear due to a lack of genetic evidence.Ī combination of genetic and biochemical studies using the yeast Saccharomyces cerevisiae, and more recently mammalian and plant systems, have begun to reveal the proteins and mechanisms behind this fundamental protein folding process. Over the past 40 yr, a number of different factors have been proposed to contribute to maintaining the oxidized environment of the ER, including the preferential secretion of reduced thiols and uptake of oxidized thiols, as well as a variety of different redox enzymes and small molecule oxidants ( Ziegler and Poulsen, 1977 Hwang et al., 1992 Carelli et al., 1997 Frand et al., 2000). Despite the ability of PDI to enhance the rate of disulfide-linked folding, how the ER disposes of electrons as a result of the oxidative disulfide formation reaction remained unknown. Studies using the classic substrate ribonuclease A led to the identification of protein disulfide isomerase (PDI), a protein that can rearrange incorrect disulfides as well as catalyze disulfide formation and reduction in vitro ( Goldberger et al., 1963). In eukaryotes, oxidative protein folding occurs in the ER. These considerations hinted that disulfide-linked folding is an assisted process in vivo, which was demonstrated by the discovery of dsbA mutants in Escherichia coli that exhibited compromised disulfide formation ( Bardwell et al., 1991). However, compared with other aspects of protein folding, disulfide-linked folding is slow due to its dependence on a redox reaction, which requires an electron acceptor. (1961) provided evidence that disulfide formation is a spontaneous process and that the polypeptide itself is sufficient for achieving the native state in vitro. These bonds are often crucial for the stability of a final protein structure, and the mispairing of cysteine residues can prevent proteins from attaining their native conformation and lead to misfolding. Proteins that traverse the secretory pathway typically depend on disulfide bonds for their maturation and function. ![]()
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