Abstract The endolysosomal network is the portal by which extracellular material enters the cell. As such, the membranes of the endosomes, phagosomes, and lysosomes that comprise this network face challenges from pathogens and other internalized materials as well as from metabolic and chemical stresses. Consequences of damage vary according to the specific compartment and degree of damage, but extensive lysosomal membrane permeabilization triggers cell death while limited disruption of endosomes and phagosomes by particulate material and pathogens leads to inflammasome activation and ensuing cytokine responses. A widely deployed strategy for removing damaged organelles involves the use of selective autophagy, referred to as lysophagy. Removal is, however, unnecessary if organelles are instead repaired. We recently discovered a new role for the ESCRT (endosomal sorting complex required for transport) machinery in responding to nano- scale disruptions in endolysosomal membranes and promoting their repair. In this project, we are building on this discovery and testing the hypothesis that ESCRTs (and in particular ESCRT-III proteins) play a key role in maintaining endolysosomal integrity and function by recognizing and repairing nanoscale membrane damage. This role for the ESCRT machinery is distinct from its widely recognized function in intralumenal vesicle biogenesis and appears applicable at both the plasma membrane and on internal organelles. Nanoscale damage involves short-lived nm-size pre-pore or pore(s) that reseal or, above a critical threshold, expand to allow unrestrained content exchange. We are using a range of chemical, physical, and biological stressors to define the signals as well as molecular and physical mechanisms underlying ESCRT-mediated repair. The equipment requested in this administrative supplement application will allow us to upgrade our existing CSU-W1 spinning disc confocal microscope system to provide uniform laser illumination along with rapid and precise photomanipulation of fluorescently tagged molecules as they respond to and protect cells from endolysosomal membrane stress. This enhanced control of illumination will enable the quantitative analyses needed to complete this project and provide insights into how ESCRTs and cooperating molecules sense and respond to a broad range of physiologic and pathophysiologic membrane stress.