Reactive oxygen species (ROS) are a significant by-product of oxidative protein folding in the endoplasmic reticulum (ER) lumen. Little is known about how cells prevent excessive oxidant accumulation and oxidant-induced damage from ER folding events. Our proposed research aims to illuminate these protective cellular mechanisms. Taking advantage of our ability to selectively create a hyper-oxidized ER, we have identified two novel redox pathways that manage ER-derived ROS. Disruption of either pathway results in a loss of cell viability upon induction of oxidative stress, suggesting that both systems are essential to maintain normal ROS physiology. Our data indicate that each pathway is unique in the means by which it influences ROS dynamics. This proposal focuses on elucidating the mechanistic features of these two important and uncharacterized cellular redox systems. In Aim 1, we will characterize a pathway that likely serves to limit cellular ROS production. This system centers on a potential electron acceptor that may compete with oxygen for electrons produced during cellular oxidative protein folding. We will focus on understanding the details of electron transfer within this system, as well as how disruption of this pathway negatively impacts cell physiology. In Aim 2, we will describe the mechanistic details for a redox-signaling pathway we identified centered on a thiol-based redox switch in the molecular chaperone BiP. We will determine how activation of the BiP-thiol switch alters BiP activity and protects cells against oxidative damage. Successful completion of the proposed studies will provide insight into the basic cell functions used to manage cellular ROS and avert cellular damage.