Lipid flip-flop in biological membranes: through the lens of TMEM16F

The asymmetric distribution of phospholipids between membrane leaflets is a hallmark of all known cells, yet many fundamental questions about membrane lipid asymmetry remain unanswered. Why do cells invest energy to establish and maintain this thermodynamically unstable state? How do cells exploit lipid asymmetry to support essential functions? What are the consequences for health and disease when this organization breaks down? Recent identification of bona fide lipid scramblases, which catalyze rapid phospholipid translocation across the hydrophobic membrane core (lipid flip-flop), has shed new light on these longstanding questions. In particular, studies of the Ca²⁺-activated phospholipid scramblase TMEM16F have revealed how regulated lipid scrambling can couple Ca²⁺ signaling to membrane remodeling, phosphatidylserine (PS) exposure, cellular communication, and disease pathogenesis. In this review, we examine the biology of regulated transmembrane phospholipid flip-flop mediated by lipid scramblases, using TMEM16F as a prototypical model to illustrate core molecular mechanisms, physiological functions, pathological implications, and emerging pharmacological opportunities. By placing TMEM16F within the broader landscape of lipid scramblases and scramblase-like proteins, we highlight how regulated membrane lipid scrambling shapes cell physiology and disease. We also discuss critical knowledge gaps and future directions for understanding this rapidly evolving field.