Supramolecular architecture of endoplasmic reticulum–plasma membrane contact sites

The endoplasmic reticulum (ER) forms membrane contact sites (MCS) with most other cellular organelles and the plasma membrane (PM). These ER–PM MCS, where the membranes of the ER and PM are closely apposed, were discovered in the early days of electron microscopy (EM), but only recently are we starting to understand their functional and structural diversity. ER–PM MCS are nowadays known to mediate excitation–contraction coupling (ECC) in striated muscle cells and to play crucial roles in Ca²⁺ and lipid homoeostasis in all metazoan cells. A common feature across ER–PM MCS specialized in different functions is the preponderance of cooperative phenomena that result in the formation of large supramolecular assemblies. Therefore, characterizing the supramolecular architecture of ER–PM MCS is critical to understand their mechanisms of function. Cryo-electron tomography (cryo-ET) is a powerful EM technique uniquely positioned to address this issue, as it allows 3D imaging of fully hydrated, unstained cellular structures at molecular resolution. In this review I summarize our current structural knowledge on the molecular organization of ER–PM MCS and its functional implications, with special emphasis on the emerging contributions of cryo-ET.