Reconstitution of human Ero1-Lalpha/protein-disulfide isomerase oxidative folding pathway in vitro. Position-dependent differences in role between the a and a' domains of protein-disulfide isomerase.

Protein-disulfide isomerase (PDI), a critical enzyme responsible for oxidative protein folding in the eukaryotic endoplasmic reticulum, is composed of four thioredoxin domains a, b, b', a', and a linker x between b' and a'. Ero1-Lalpha, an oxidase for human PDI (hPDI), has been determined to have one molecular flavin adenine dinucleotide (FAD) as its prosthetic group. Oxygen consumption assays with purified recombinant Ero1-Lalpha revealed that it utilizes oxygen as a terminal electron acceptor producing one disulfide bond and one molecule of hydrogen peroxide per dioxygen molecule consumed. Exogenous FAD is not required for recombinant Ero1-Lalpha activity. By monitoring the reactivation of denatured and reduced RNase A, we reconstituted the Ero1-Lalpha/hPDI oxidative folding system in vitro and determined the enzymatic activities of hPDI in this system. Mutagenesis studies suggested that the a' domain of hPDI is much more active than the a domain in Ero1-Lalpha-mediated oxidative folding. A domain swapping study revealed that one catalytic thioredoxin domain to the C-terminal of bb'x, whether a or a', is essential in Ero1-Lalpha-mediated oxidative folding. These data, combined with a pull-down assay and isothermal titration calorimetry measurements, enabled the minimal element for binding with Ero1-Lalpha to be mapped to the b'xa' fragment of hPDI.