Membrane Contact Site

Membrane Contact Site

Membrane contact sites (MCS) are close appositions between two organelles. Ultrastructural studies typically reveal an intermembrane distance in the order of the size of a single protein (10 nm). These zones of apposition are highly conserved in evolution but not much is understood about their biological meaning. These sites are thought to be important in mainly three cellular functions: they facilitate signalling, they promote the passage of ions, and they are the sites where the non-vesicular lipid trafficking from one cellular compartment to another occurs. MCS may be particularly important in the function of the endoplasmic reticulum (ER), since this is the major site of lipid synthesis within cells. These structures can form between the ER and many organelles, including mitochondria, Golgi, endosomes, lysosomes, peroxisomes, chloroplasts and the plasma membrane. They can also form between other organelles, such as between the cell nucleus and the vacuole in yeast (nucleus vacuole junction, NVJ). First mentions of these contact sites can be found in papers published in the late 1950s mainly visualized using electron microscopy (EM) techniques. Copeland and Dalton described them as “highly specialized tubular form of endoplasmic reticulum in association with the mitochondria and apparently in turn, with the vascular border of the cell”.