The defining feature of eukaryotic cells is the presence of membrane-bound organelles of diverse kinds, each with specialized functions. Most organelles have multiple copies in cells. By contrast, each cell contains only one endoplasmic reticulum (ER). However, the ER consists of an elaborate network of membrane cisternae and tubules that extends throughout the cell and occupies a large fraction of the cytoplasmic volume. Though compartmentalization of biochemical reactions and processes in these organelles has obvious advantages, it also poses challenges for their coordinated activity, requiring mechanisms for regulated interorganelle communication. However, these mechanisms have remained elusive, and the quintessential textbook diagram still pictures organelles in isolation, floating in a cytoplasmic sea. The last decade has radically changed this view, and membrane contact sites (MCSs) between different organelles have been brought to center stage as prime, highly regulated routes for interorganelle communication essential for cell homeostasis.

The presence of organelle contacts was recognized long ago. However, the functions of these structures remained unclear. Recent advances in the resolution of microscopy and the development of unique fluorophores have markedly advanced our ability to study interorganelle MCSs. The three-dimensional structure of ER MCSs with other organelles and the plasma membrane (PM) can be visualized at nanometer resolution by electron microscopy (EM). Multispectral live-cell fluorescence microscopy displays the behavior of MCSs over time and in response to stimuli. Together, these data have revealed the general features of MCSs. For example, EM has revealed that MCSs are closely opposed and tethered but not fused membranes, MCSs are spaced at 10 to 30 nm, and ribosomes are excluded from the ER surface at these sites. Fluorescence microscopy demonstrates that organelles can remain attached to ER tubules as they traffic along microtubules. The combinations of these tools with classical molecular biology and biochemical tools have identified molecules implicated in several MCSs and elucidated their functions, including lipid and ion transport between organelles and organelle positioning and division.

MCSs are central to normal cell physiology. Moreover, several MCS proteins are linked to various diseases: seipin, protrudin, and spastin to hereditary spastic paraplegia; VAPA and VAPB to amyotrophic lateral sclerosis; Dnm2 and Mfn2 to Charcot-Marie-Tooth disease; STIM1 and Orai1 to tubular aggregate myopathy; and ACBD5 to retinal dystrophy. Whether defects in MCS functions cause these diseases directly or indirectly remains to be explored. Recent progress has begun to identify some of the molecular machineries that regulate MCS formation. Dissecting the roles of these factors will strengthen our understanding of the integrative nature of MCSs. The advancement of diverse microscopy techniques will allow us to track multiple factors at MCSs simultaneously in real time and at high resolution, and this may help us gain a more detailed view of the biology of MCSs and their related physiological processes.

The ER forms MCSs with mitochondria, Golgi, endosomes, peroxisomes, lipid droplets, and the PM. These MCSs are closely opposed but not fused membranes containing various molecular machineries. Factors localized to these MCSs mediate essential cellular processes, including lipid and ion exchange, organelle positioning, and biogenesis.