1.R.1. The Membrane Contact Site (MCS) Family
Membrane contact sites (MCSs), or Organelle contact zones, form junctions between organelles. Phospholipids are synthesized in the endoplasmic reticulum (ER), the largest membrane bound organelle that forms 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. Their localizations offer 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). The PDZD8 protein interacts with Protrudin (1.R.1.1.1) and Rab7 (9.A.3.1.1) at ER-late endosome membrane contact sites which also associate with mitochondria (a 3-way contact site). (Elbaz-Alon et al. 2020). Thus, PDZD8 is a shared component of two distinct MCSs, suggesting a role for SMP-mediated lipid transport in the regulation of endosome function. Another mitochondrial outer membrane constituent constituent of MCSs is mitoguardin 2 (MIGA2; FAM73B) of 593 aas and possibly 4TMSs, 3 N-terminal, and 1 near the C-terminus. MIGA proteins function downstream of mitofusin and interact with MitoPLD to stabilize MitoPLD and facilitate MitoPLD dimer formation. Thus, MIGA proteins promote mitochondrial fusion by regulating mitochondrial phospholipid metabolism via MitoPLD. Additional proteins not listed under TC# 1.R.1 but possibly playing a role in transfer between organelles are: (1) lipid transfer proteins (LTPs; TC# 8.A.120), (2) oxysterol binding proteins (OSBPs; TC# 2.D.1), and (3) PLD6 Q8N2A8) (endonuclease, homologous to cardiolipin hydrolases (252 aas and 2 - 4 TMSs in the N-terminal half of the protein; PLD6 localizes to the outer mitochondrial membrane facing the cytosol (Huang et al. 2011) and has been shown to be a backbone-non-specific, single strand-specific nuclease, cleaving either RNA or DNA substrates with similar affinity.)
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). Organellar membrane tethering sites/factors include ERMES (ER-mitochondrial encounter structures), NVJs (Nuclear-vacuole jonctions), vCLAMP (Vacuole and mitochndrial patch), and MICOS (Mitochondrial contact sites) (Tamura et al. 2018). Mitofusins, components of MCSs, can assume a topology which places the redox-regulated C terminus in the mitochondrial intermembrane space (Mattie et al. 2018). Mitofusins include, in addition to their GTPase and transmembrane domains, two heptad repeat domains, HR1 and HR2. All four regions are crucial for mitofusin function. Cohen and Tareste 2018 reviewed strategies employed by various protein machineries distinct from Mitofusins to mediate membrane fusion. They then present recent structure-function data on Mitofusins that provide insights into their mode of action in mitochondrial fusion.
Vesicle fusion involves vesicle tethering, docking, and membrane merger. Koshiba et al. 2004 showed that mitofusin is required on adjacent mitochondria to mediate fusion, indicating that mitofusin complexes act in trans (between adjacent mitochondria). A heptad repeat region (HR2) mediates mitofusin oligomerization by assembling a dimeric, antiparallel coiled coil. The TMSs are located at opposite ends of the 95 Å coiled coil and provide a mechanism for organelle tethering (Koshiba et al. 2004).
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).
Lipids are stored in lipid droplets in adipocytes, and these lipid droplets interact with mitochondria and the ER. The outer mitochondrial membrane protein, MIGA2, tethers lipid droplets to mitochondria, and interacts also with two ER membrane proteins, VAPA and VAPB. MIGA2 is required for the de novo synthesis of lipids and links mitochondrial lipogenesis and ER triglyceride syntehsis during lipd-droplet loading (Freyre et al. 2019). MIGA2 is also a regulator of mitochondrial fusion: it acts by forming
homo- and heterodimers at the mitochondrial outer membrane and
facilitates the formation of PLD6/MitoPLD dimers. It may also act by regulating
phospholipid metabolism via PLD6/MitoPLD (Zhang et al. 2016).
A key characteristic of eukaryotic cells is the presence of organelles with discrete boundaries and functions. Such subcellular compartmentalization into organelles necessitates platforms for communication and material exchange between each other which often involves vesicular trafficking and associated processes. Another way is via the close apposition between organellar membranes via MCSs. Apart from lipid transfer, MCSs have been implicated in the mediation of various cellular processes including ion transport, apoptosis, and organelle dynamics (Santos and Nozaki 2021). In mammalian and yeast cells, contact sites have been reported between the membranes of the following: the endoplasmic reticulum (ER) and the plasma membrane (PM), ER and the Golgi apparatus, ER and endosomes (i.e., vacuoles, lysosomes), ER and lipid droplets (LD), the mitochondria and vacuoles, the nucleus and vacuoles, and the mitochondria and lipid droplets
(Santos and Nozaki 2021).