1) ( 1, 2, 4, 5).ĭiagram of the domain structures of BAR-, EFC/F-BAR- and IMD/I-BAR -domain-containing proteins. For example, the SH3 domains also bind to N-WASP/WAVE proteins, regulator of actin cytoskeleton and dynamin, a molecule that pinches the membrane into vesicles, as described in several reports ( Fig. These BAR domain superfamily proteins not only deform membranes but also have several other functional domains. BAR domain superfamily proteins deform membranes to a geometry that corresponds to the structures of the membrane-binding surface of the proteins, and/or bind to the membranes that fit their structures, and thus function to generate specific membrane geometries ( 1–3). These proteins belong to the BAR domain superfamily, and contain the Bin-Amphiphysin-Rvs167 (BAR), extended Fes-CIP4 homology (EFC)/FCH-BAR (F-BAR) and IRSp53-MIM homology domain (IMD)/inverse-BAR (I-BAR) domains, which have structurally determined membrane contact surfaces that are considered to function in membrane curvature formation, recognition and maintenance. Recently, increasing numbers of membrane-deforming proteins connecting the actin cytoskeleton and the plasma membrane have been found. However, it has been unclear how these characteristic structures are formed by the dynamic collaboration between the membrane binding proteins, the actin cytoskeleton and the plasma membrane. In most cases, actin filaments accompany these structures. Each of these fine micro-membrane structures has its specific morphological characteristics, which have been mainly defined by extensive electron micrographic studies. These fine structures of the plasma membrane include invaginations, such as caveolae and clathrin-coated pits, as well as protrusions, such as filopodia and lamellipodia. The plasma membrane contains various membrane microstructures, ranging in size from ∼10 nm to few microns, which are supported by the cytoskeleton and/or membrane binding proteins. These membrane tubulations are also considered to function in membrane fusion and fission.Īctin cytoskeleton, BAR domain, EFC domain, F-BAR domain, I-BAR domain, IMD domain, membrane curvature, membrane fission, membrane fusion, plasma membrane Importantly, the SH3 domains of the BAR domain superfamily proteins bind to the actin regulatory WASP/WAVE proteins, and the BAR/F-BAR/I-BAR domain-SH3 unit could orient the actin filaments towards the membrane for each subcellular structure. The BAR domain superfamily proteins have structurally determined positive and negative curvatures of membrane contact at their BAR, F-BAR and I-BAR domains, which generate and maintain such curved membranes by binding to the membrane. There are six modes of membrane curvature at the plasma membrane, which are classified by the positive or negative curvature and the location of the curvature (tip, neck or shaft of the tubules). These structures of the plasma membrane can be considered as tubules or inverted tubules (protrusions) of the membrane. The plasma membrane, the outermost surface of eukaryotic cells, contains various substructures, such as protrusions or invaginations, which are associated with diverse functions, including endocytosis and cell migration.
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