The maintenance of proper protein folding and function in the cell (proteostasis) relies on many integrated and highly regulated processes. A dysregulation in proteostasis is a major contributing factor in diseases like cancer, diabetes, and Alzheimer’s. To help promote protein folding, cells use chaperone proteins to ensure nascent polypeptides reach their native, folded, and functional state. Chaperones also help to clear misfolded proteins from the cell to prevent their accumulation. Most chaperones are ATPases that use the energy from nucleotide hydrolysis to drive the binding and release of folding and/or misfolded polypeptides. The molecular chaperone BiP is localized to the folding environment of the endoplasmic reticulum (ER), where it not only helps proteins fold under normal growth conditions but also acts to limit protein aggregation during conditions of elevated reactive oxygen species (ROS), which can cause protein damage, misfolding, and aggregation. Sil1 is a co-chaperone specific for BiP and is responsible for modulating BiP activities in three different ways. As a nucleotide exchange factor, Sil1 removes ADP from BiP and promotes the release of polypeptides from BiP and the continuation of BiP’s ATP-dependent chaperone cycle. Sil1 also prevents the reassociation of released polypeptides, which prevents the extended engagement of BiP with substrates. Additionally, Sil1 acts as a reductase to reduce (remove) a redox-modification on BiP that shifts the activity of BiP during high levels of ROS, changing BiP from an ATP-dependent foldase to a holdase that can limit protein aggregation during the unfavorable folding environment associated with high ROS. While the nucleotide exchange activity of Sil1 has been the subject of many studies, the peptide-release and reductase activities are less understood. I plan on investigating these activities of Sil1 using two, complementary, approaches. First, I aim to understand the physical contacts that form between Sil1 and BiP to inform on the mechanisms for Sil1-mediated peptide release and reductase activities. These studies will be complemented by a biochemical analysis of how the different Sil1 activities work together to promote BiP’s chaperone cycle. Together, my two aims will make for a comprehensive analysis of how perturbations in the regulation of a major cellular chaperone affects cellular proteostasis, and thus organismal health and function.