What is the function of AP2 in clathrin coated pits?
What are clathrin coated pits used for? Function. Clathrin performs critical roles in shaping rounded vesicles in the cytoplasm for intracellular trafficking. Clathrin-coated vesicles (CCV) selectively sort cargo at the cell membrane, trans-Golgi network, and endosomal compartments for multiple membrane traffic pathways. Click to see full answer.
Where are clathrin coated pits located in the plasma membrane?
Living Clathrin-Coated Pits. Traffic 2004; 5: 327–337 329. The rapid vanishing of plasma membrane associated clathrin spots (CS) was interpreted as the direct observation of CCV formation and transport deeper in the cytosol, leaving the focal plane, and/or loss of their clathrin coat (uncoating reac- tion).ThemoststrikingresultwasthatCCPsseemedtoform in …
What is the function of clathrin coated vesicles?
Coated pits are regions of the cell membrane specialized in receptor-mediated endocytosis. Their cytoplasmic surface is coated with a bristlelike structure made of clathrin. During the first steps of endocytosis, clathrin-coated pits are internalized to form clathrin-coated vesicles which transport proteins from organelle to organelle. Category
What is the role of clathrin-coated pits in endocytosis?
Clathrin is involved in coating membranes that are endocytosed from the plasma membrane and those that move between the trans-Golgi network (TGN) and endosomes . When coating membranes, clathrin does not link to the membrane directly, but does so via adaptor proteins. The main class of adaptors are the adaptor protein (AP) complexes . AP-2 and AP-1 are members …
What is the function of clathrin?
Clathrin is a self-assembling protein that is recruited to membranes from the cytoplasm of eukaryotic cells to form a protein coat. The function of this coat is to sort proteins in the membrane and to contribute to membrane deformation.
Are clathrin coated pits used in phagocytosis?
Clathrin has been implicated as a necessary component of phagocytosis (15). Clathrin-coated pits are found in peritoneal macrophages (14, 16), and are located at surface adhesion sites (17) and phagosomes (18) in the macrophage.
What is the role of clathrin in endocytosis?
Clathrin-mediated endocytosis is the endocytic portal into cells through which cargo is packaged into vesicles with the aid of a clathrin coat. It is fundamental to neurotransmission, signal transduction and the regulation of many plasma membrane activities and is thus essential to higher eukaryotic life.Jul 22, 2011
What is the purpose of the clathrin coat quizlet?
Clathrin-coated vesicles mediate transport from the Golgi apparatus and from the plasma membrane (from the plasma membrane and between endosomal and Golgi compartments), whereas COPI- and COPII-coated vesicles most commonly mediate transport from the ER and from the Golgi cisternae (both COPI- and COPII-coated vesicles ...
What is the role of clathrin in vesicle formation?
Clathrin performs critical roles in shaping rounded vesicles in the cytoplasm for intracellular trafficking. Clathrin-coated vesicles (CCV) selectively sort cargo at the cell membrane, trans-Golgi network, and endosomal compartments for multiple membrane traffic pathways.
What is a coated pit in biology?
Definition. Coated pits are regions of the cell membrane specialized in receptor-mediated endocytosis. Their cytoplasmic surface is coated with a bristlelike structure made of clathrin.
What do clathrin-coated vesicles carry?
Clathrin-coated vesicles were the first discovered and remain the most extensively characterized transport vesicles. They mediate endocytosis of transmembrane receptors and transport of newly synthesized lysosomal hydrolases from the trans-Golgi network to the lysosome.
How are clathrin coated pits and caveolae similar?
Caveolae and clathrin-coated vesicles are both specialized regions of the plasma membrane, crucial to the endomembrane system within the cell. They are involved in the internalization of proteins and lipids, as well as other membrane trafficking between cellular organelles.
What does clathrin mediated endocytosis require?
Initiation of the clathrin complex formation requires the accumulation of phosphatidylinositol‑4,5‑bisphosphate (PIP2) and adaptor proteins, such as AP-2, at the pinching site [6][7][8]. In the case of clathrin-coated vesicles (CCV) formed at the trans-Golgi apparatus (TGA), AP-1 is essential [9][10].
Where are Clathrin coated pits located?
Clathrin-coated pits are normally restricted to the region of the plasma membrane by the cortical cytoplasm actin organization.
How long does it take for a clathrin coated pit to form?
Ligand-receptor complexes concentrate in coated pits on the cell surface and then pinch off to form clathrin-coated vesicles that carry the cargo into the cell, completing the budding process in approximately 1 minute. Coated pits typically occupy 1% to 2% of the plasma membrane surface area.
What happens to GTP hydrolysis during interactions between turns of the dynamin spiral?
GTP hydrolysis during interactions between turns of the dynamin spiral constricts the collar and promotes scission of the membrane. In cells without dynamin, vesicle formation arrests at the stage of clathrin coat formation or before vesicle scission.
How are CCVs formed?
CCVs are formed by the coordinated assembly of clathrin triskelia built from three tightly linked heavy and associated light chains onto the plasma membrane. The recruitment and polymerization of the outer clathrin layer is assisted by mono- and heterotetrameric adaptor proteins, which simultaneously bind to clathrin, to membrane lipids, and in many cases to transmembrane cargo proteins. In addition, there is a large reservoir of preassembled flat hexagonal clathrin lattices at the plasma membrane that, however, need to undergo a structural transition involving the formation of clathrin pentagons in order to accommodate a curved membrane bud. The most important clathrin adaptor is the heterotetrameric AP-2 complex comprising two large subunits ( α and β 2), a medium subunit ( μ 2), and a small subunit ( σ 2). The two large subunits together with σ 2 and the amino-terminal domain of μ 2 (N- μ 2) form the trunk or core domain of AP-2, and are joined by extended, flexible ‘hinges’ to the appendage or ear domains of α - and β 2-adaptins. Since AP-2 associates with clathrin, a variety of accessory endocytic proteins, phosphatidylinositol 4,5-bisphosphate [PI (4,5)P 2 ], and membrane cargo proteins, it has been postulated to serve as a main protein interaction hub during coated pit assembly. Many accessory proteins, such as epsins, AP180/CALM, and amphiphysin, also have an adaptor function by linking clathrin assembly to membrane bud formation. These mono- or dimeric adaptors possess a folded lipid-binding domain linked to a more flexible portion of the protein harboring short clathrin- and AP-2-binding motifs, which may aid stabilization of nascent clathrin-coated pits (CCPs) during the assembly process. During CCP assembly transmembrane cargo proteins are recognized by adaptor proteins, most notably the AP-2 complex, which bind to endocytic sorting motifs within their cytoplasmic tails. These motifs include tyrosine-based Yxxø (where ø is a bulky hydrophobic residue) and acidic cluster di-leucine motifs, which bind directly to distinct sites within the AP-2 core domain. Yxxø motifs have been co-crystallized with the carboxy-terminal portion of the AP-2 μ -subunit (C- μ 2), to which they bind in an extended conformation. Cargo recognition by AP-2 requires the presence of PI (4,5)P 2, which stabilizes the protein in an open conformation that enables cargo recognition by its μ 2-subunit. Consistent with this, clathrin/AP-2-coated pits were shown to become stabilized in living cells upon encounter of cargo receptors, suggesting that the process of AP-2 recruitment and initiation of plasmalemmal CCPs is highly cooperative.
What is clathrin-dependent endocytosis?
Clathrin-dependent endocytosis ( Fig. 22.8) occurs on specialized patches of the plasma membrane, called coated pits, formed by a protein lattice of clathrin and adapter molecules on their cytoplasmic surface (see Fig. 21.12 for details about clathrin structure and mechanism). Eukaryotic cells use clathrin-mediated endocytosis to obtain essential nutrients, such as iron and cholesterol, and to remove activated receptors from the cell surface. The process also controls the activation of signaling pathways and participates in the turnover of membrane components. Clathrin-coated vesicles also retrieve synaptic vesicle membrane at synapses following neurotransmitter release (see Fig. 17.9 ). In addition to its role in endocytosis at the plasma membrane, clathrin also participates in cargo sorting and membrane budding at other sites in cells, including endosomes and the trans -Golgi network (TGN).
What is the role of clathrin in endocytosis?
In addition to its role in endocytosis at the plasma membrane, clathrin also participates in cargo sorting and membrane budding at other sites in cells , including endosomes and the trans -Golgi network (TGN).
How big is a CCV?
The size of CCVs can range from 60 to 200 nm in diameter ( Pearse and Crowther, 1987 ). At the neuronal synapse, where clathrin is involved in recapture of synaptic vesicle proteins, CCV size is restricted to the smaller dimensions in this range by the neuronal assembly protein AP180 ( Morris et al., 1993 ).
What are caveolae in a cell?
Caveolae are complex plasma membrane structures whose properties appear to place them between coated pits and lipid rafts (Chapter 8 ). They are small (50–100 nm) invaginated membrane structures that superficially resemble coated pits ( Fig. 11.9, [32] ). In fact, in 1955, Yamada [33] proposed the descriptive name “caveolae” which is Latin for little caves. Caveolae were first described by the electron microscopist George Palade in 1953 and are abundant in many vertebrate cell types, especially endothelial cells and adipocytes where they may account for 30–70% of the total plasma membrane surface area. Caveolae however, are not a universal feature of all cells as they are totally absent in neurons. Like lipid rafts, caveolae are partially characterized by being enriched in sphingolipids and cholesterol and also participate in signal transduction processes [34,35]. In fact caveolae are often described as being “invaginated lipid rafts” that differ primarily by the presence of a family of marker proteins called caveolins. But, similar to coated pits, caveolae may also play a role in endocytosis.
What are non-visual arrestins? What are their functions?
The role of non-visual arrestins in recruiting GPCRs to coated pits and facilitation of receptor internalization via this pathway is fairly well established. The case of ubiquitin modification of receptors and arrestins is less straightforward: arrestins seem to recruit enzymes responsible for ubiquitination and deubiquitination of GPCRs. These modifications play distinct roles in receptor trafficking, but the exact role of non-visual arrestins, which are also ubiquitinated in response to receptor stimulation, remains to be elucidated. The functions of non-visual arrestins in complex trafficking itineraries of individual GPCR subtypes might be different. How arrestins affect the recycling of internalized GPCRs, and how exactly arrestin binding regulates NSF function and vesicle trafficking, remains even less clear (Fig. 1 ). Cytoskeleton is intimately involved in trafficking of many proteins. Arrestins were shown to bind microtubules 80–82 and a very specialized structure containing polymerized tubulin, the centrosome. 83 However, the role of these interactions in the transport of receptors and/or other molecules within the cell still needs to be defined. Most likely, recent finding that non-visual arrestins recruit clathrin to microtubules targeting focal adhesions, thereby facilitating integrin internalization and focal adhesion disassembly, 84 is only the tip of the iceberg.
How does endocytosis work?
Endocytosis involves the capture of material from the surface of a cell and transport of that material into the cytoplasm. Material is brought into the cell inside membrane-bound endocytic vesicles that are formed from the phospholipid bilayer of the cell’s plasma membrane (PM). Imagine the PM as the surface of a balloon. Pushing a finger into the balloon’s surface will cause the membrane to bend forming an indentation or pit. But a membrane, like a balloon, cannot change shape without force being applied. Peripheral membrane proteins present on the cytoplasmic side of the membrane are thought to produce the force necessary for inward bending of the PM.
How do brain ECs work?
Brain ECs possess insulin receptors, which aggregate in “coated pits” on the plasma membrane and can be used to deliver therapeut ics. As insulin molecules in the circulation bind to these receptors, the pits invaginate and form coated vesicles. The endosome then acidifies and the insulin dissociates from the receptor and traverses the membrane.87,89 This efficient transport is evidenced by the fact that insulin mRNA is not present in the brain, but insulin itself is present in the brain and in the periphery derived from blood. 90-92
How does internalization of PDGF receptors work?
Internalization of the PDGF receptors, mediated by clathrin-coated pits, occur s shortly after ligand stimulation and is dependent on the receptor kinase activity. Intracellular trafficking of the PDGF β -receptor involves PI3-kinase, but is not entirely elucidated yet. Internalized receptors can be recycled and go back to the cell surface or can be directed to lysosomal degradation, which actually results in receptor downregulation. However, before they are degraded, internalized receptors remain associated with downstream proteins and continue to signal. For this reason, internalization and subsequent trafficking can be considered a late signaling stage rather than merely the method of receptor inactivation.
What is clathrin coated pit?
Clathrin coated pits are specialized patches at the plasma membrane that concentrate receptors, curve to form an invagination and bud off with their receptor cargo in the process of clathrin mediated endocytosis (CME) ( Robinson, 2015 ). CME is the main route of receptor internalization in mammalian cells ( Bitsikas et al., 2014, Watts and Marsh, 1992) and this well conserved mechanism has been intensively studied for over 40 years (reviewed in Brodsky, 2012, Robinson, 2015 ). Before the advent of fluorescent proteins a combination of biochemistry, immunofluorescence and electron microscopy (EM) was used to infer a time-line of clathrin coated pit nucleation, coat formation, inward invagination and budding ( Brodsky, 2012, Robinson, 2015 ). However, nearly two decades ago fluorescent proteins were introduced and the dynamics of CME could then be analysed in live cells using fluorescence microscopy ( Gaidarov et al., 1999 ). The application of epifluorescence ( Gaidarov et al., 1999 ), spinning disc confocal fluorescence ( Ehrlich et al., 2004) and total internal reflection fluorescence microscopy (TIRFM) ( Merrifield et al., 2002) prompted a rapid expansion of imaging studies and yielding new insights into the detailed molecular dynamics of CME in mammalian cells.
What is the role of clathrin plaques in cell adhesion?
In man-made structures the problem of fixing a thin, flexible skin to a rigid substrate (for instance the aluminium ‘skin’ of an aircraft to the rigid fuselage substructure) traditionally uses rivets (equivalent to the cells’ adhesion molecules) and in areas of high stress the thin skin (viz the plasma membrane) is stabilized, and mechanical loads distributed more evenly, using a ‘backing plate’ or ‘doubler’ ( viz a clathrin plaque). Clathrin plaques may act in a similar way as a ‘doubler’ to distribute mechanical load to the plasma membrane and cortical actin ctyoskeleton , as well as integrating the adhesion of otherwise dispersed adhesion molecules.
How does clathrin form a membrane?
Clathrin forms a membrane ‘coat’ when clathrin triskelia interlink to form a highly organized hexagonal lattice, bound to the plasma membrane and receptor cargo by adaptor proteins. To accommodate curvature pentagons must be incorporated in the otherwise hexagonal clathrin lattice as the coat polymerizes ( den Otter and Briels, 2011 ). Clathrin coated pit nucleation is thought to begin with the chance encounter of phosphatidylinositol-2-bisphosphate (PIP2), the adaptor protein complex AP2 and clathrin triskelia ( Cocucci et al., 2012, Heuser, 1980, Kelly et al., 2014 ). Single molecule imaging experiments suggested that the minimal nucleation complex for CCP formation consists of one clathrin triskelia and two AP2 molecules ( Fig. 1 A) ( Cocucci et al., 2012 ). The curved clathrin coat is then understood to propagate and grown through the recruitment of additional cargo/adaptor/clathrin at the edges of the growing coat ( Fig. 1 B).
What is clathrin mediated endocytosis?
Clathrin mediated endocytosis (CME) is the main route of receptor internalization in mammalian cells and this well conserved mechanism has been intensively studied for over 40 yrs. In the general or ‘canonical’ model of CME clathrin coated pits form stochastically at the plasma membrane and coated pit curvature develops as the coated pit grows through clathrin polymerization. However, the canonical model of CME does not explain the diversity of endocytically active clathrin coated structures (CCSs) found at the plasma membrane by both electron and light microscopy. In this review we examine the canonical model of CME, highlight discrepancies with published experimental data and suggest future avenues of exploration while paying particular attention to the relationship between clathrin coated pits, plaques, sites of adhesion and the formation of endocytic ‘hotspots’.