[Functions and measurement methods of organelle contact sites]

Intracellular organelles do not function in isolation but instead cooperate through organelle contact sites, where membranes closely appose without fusion to exchange information and metabolites. Among these interfaces, mitochondria-endoplasmic reticulum contact sites (MERCs) have emerged as central regulatory hubs involved not only in calcium and lipid exchange but also in mitochondrial dynamics, autophagy, stress responses, cell death, and metabolic regulation. In this review, the molecular basis of MERC formation is first organized from the perspectives of tethering, molecular transfer, and contact-site regulation, emphasizing that MERCs represent dynamic functional domains that are reorganized according to cellular conditions rather than static structures. Our recent findings are then introduced demonstrating that the mitochondrial outer membrane E3 ubiquitin ligase MITOL (also known as MARCHF5) selectively modulates substrate activity at MERCs and may contribute to mitochondrial iron supply and respiratory maintenance through regulation of the heme-degrading enzyme HMOX2. Because MERCs undergo rapid and reversible remodeling, quantitative analysis in living cells is essential. A split-luciferase-based reversible assay is presented as an example of an approach for real-time monitoring of MERC dynamics, revealing a stress-responsive increase in MERCs triggered by mitochondrial reactive oxygen species that is linked to the handling of lipid radicals. Finally, current methodologies for MERC analysis, including electron microscopy, super-resolution imaging, proximity sensors, and proximity labeling, are overviewed.