The MCSs formed between the ER and the PM provide an ideal platform for non-vesicular transport of lipids, ions, and many other signaling molecules ( Gallo et al., 2016 Saheki and De Camilli, 2017a Stefan, 2020). ER-PM contact sites were first observed in muscle cells in the 1950s ( Porter and Palade, 1957), and later were demonstrated as a general feature in eukaryotes. The ER extends throughout the whole cell and engages in broad communications with PM and other organelles by MCSs. It is the primary place for the synthesis of proteins and lipids, which are needed to maintain and propagate other membranous organelles and plasma membrane (PM) ( Bonifacino and Glick, 2004). The largest membrane-bound organelle in eukaryotic cells is the endoplasmic reticulum (ER). This kind of intracellular communication is ensured by particular regions within the cell, defined as membrane contact sites (MCSs), structures mediated by protein-protein or protein-lipid interactions. However, recent studies demonstrated that non-vesicular trafficking is another critical trafficking approach among intracellular membranous organelles, directly communicating through a close gap (typically within 10–30 nm) formed by two opposed membranes ( Wong et al., 2019).
The cargo is wrapped by or integrated into the membrane to form a vesicle, and exchange proteins or lipids between organelles through membrane fusion ( Bonifacino and Glick, 2004 Südhof and Rothman, 2009). Vesicular trafficking is the predominant pathway to transport macromolecular substances and exchange information between organelles. Intracellular trafficking between membrane-bound organelles is divided into two types, vesicular trafficking and non-vesicular trafficking. This review focuses on proteins functioning at ER-PM contact sites and highlights the recent progress in their mechanisms and physiological roles. The physiological roles of ER-PM contact sites are dependent on a variety of regulators that individually or cooperatively perform functions in diverse cellular processes. Distinct tethering factors dynamically control the architecture of ER-PM junctions in response to intracellular signals or external stimuli. These ER-PM contact sites play essential roles in lipid homeostasis, ion dynamics, and cell signaling, which are carried out by protein-protein or protein-lipid interactions. The endoplasmic reticulum (ER) forms direct membrane contact sites with the plasma membrane (PM) in eukaryotic cells. 2Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, CO, United States.1Jiangsu Key Laboratory for Molecular and Medical Biotechnology, College of Life Sciences, Nanjing Normal University, Nanjing, China.Chenlu Li 1 Tiantian Qian 1 Ruyue He 1 Chun Wan 2 Yinghui Liu 1 * Haijia Yu 1 *
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