Significance The current study reveals the functions of FAF1 in protecting cells from ferroptosis, a novel cell death pathway triggered by PUFA peroxidation. In the absence of FAF1, cultured cells and mice are extremely sensitive to ferroptosis when exposed to physiological levels of PUFAs. Mechanistically, FAF1 sequesters PUFAs into the hydrophobic core of a global structure that limits their access to positively charged Fe 2+ , which catalyzes the peroxidation reaction. These observations suggest that FAF1-mediated protection of PUFA peroxidation plays a critical role in preventing initiation of ferroptosis.

Fatty acids (FAs) are well known to serve as substrates for reactions that provide cells with membranes and energy. In contrast to these metabolic reactions, the physiological importance of FAs themselves known as free FAs (FFAs) in cells remains obscure. Since accumulation of FFAs in cells is toxic, cells must develop mechanisms to detoxify FFAs. One such mechanism is to sequester free polyunsaturated FAs (PUFAs) into a droplet‐like structure assembled by Fas‐Associated Factor 1 (FAF1), a cytosolic protein. This sequestration limits access of PUFAs to Fe2+, thereby preventing Fe2+‐catalyzed PUFA peroxidation. Consequently, assembly of the FAF1‐FFA complex is critical to protect cells from ferroptosis, a cell death pathway triggered by PUFA peroxidation. The observations that free PUFAs in cytosol are not randomly diffused but rather sequestered into a membraneless complex should open new directions to explore signaling pathways by which FFAs regulate cellular physiology.