1.R.1. The Membrane Contact Site (MCS) Family
Membrane contact sites form junctions between organelles. Phospholipids are synthesized at the endoplasmic reticulum (ER), the largest membrane bound organelle that forms membrane contact sites (MCSs) with almost every other organelle. MCSs are locations at which the membranes of two organelles are closely positioned to provide a microenvironment where proteins in one membrane can interact with those in the opposite membrane. Thus, MCSs provide a location at which lipid transfer proteins (LTPs) can achieve the efficient transfer of individual classes of lipids from the ER to other organelles via non-vesicular transport. Cockcroft and Raghu 2018 described the localization and biochemical activity of LTPs at MCSs between the ER and other cellular membranes. The localization of LTPs at MCSs offers an elegant cell biological solution to tune local lipid composition to ongoing cell physiology. LTPs are mediators of lipid transport from the ER to other organelles; inter-organellar transport occurs at MCSs in a nonvesicular manner (Hanada 2018).
MCSs are sites of close apposition between two or more organelles that play diverse roles in the exchange of metabolites, lipids and proteins. Moreover, the biogenesis of autophagosomes and peroxisomes involves contributions from the ER and multiple other cellular compartments (Cohen et al. 2018). Cellular organelles form multiple junctional complexes with one another and facilitate transfer of calcium, sterols, phospholipids, iron and possibly other substances between the organelles. Mitochondrial junctions, joining mitochondria with other organelles, are concerned with Ca2+ signaling (Pietrangelo and Ridgway 2018).
Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a large eukaryotic gene family that transports and regulates the metabolism of sterols and phospholipids. The original classification of the family based on oxysterol-binding activity belies the complex dual lipid-binding specificity of the conserved OSBP homology domain (OHD). Additional protein- and membrane-interacting modules mediate the targeting of select OSBP/ORPs to membrane contact sites between organelles, thus positioning the OHD between opposing membranes for lipid transfer and metabolic regulation. This unique subcellular location, coupled with diverse ligand preferences and tissue distribution, has identified OSBP/ORPs as key arbiters of membrane composition and function (Pietrangelo and Ridgway 2018).