Clathrin-mediated endocytosis (Homo sapiens)
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Description
Clathrin-mediated endocytosis (CME) is one of a number of process that control the uptake of material from the plasma membrane, and leads to the formation of clathrin-coated vesicles (Pearse et al, 1975; reviewed in Robinson, 2015; McMahon and Boucrot, 2011; Kirchhausen et al, 2014). CME contributes to signal transduction by regulating the cell surface expression and signaling of receptor tyrosine kinases (RTKs) and G-protein coupled receptors (GPCRs). Most RTKs exhibit a robust increase in internalization rate after binding specific ligands; however, some RTKs may also exhibit significant ligand-independent internalization (reviewed in Goh and Sorkin, 2013). CME controls RTK and GPCR signaling by organizing signaling both within the plasma membrane and on endosomes (reviewed in Eichel et al, 2016; Garay et al, 2015; Vieira et al, 1996; Sorkin and von Zastrow, 2014; Di Fiori and von Zastrow, 2014; Barbieri et al, 2016). CME also contributes to the uptake of material such as metabolites, hormones and other proteins from the extracellular space, and regulates membrane composition by recycling membrane components and/or targeting them for degradation.
Clathrin-mediated endocytosis involves initiation of clathrin-coated pit (CCP) formation, cargo selection, coat assembly and stabilization, membrane scission and vesicle uncoating. Although for simplicity in this pathway, the steps leading to a mature CCP are represented in a linear and temporally distinct fashion, the formation of a clathrin-coated vesicle is a highly heterogeneous process and clear temporal boundaries between these processes may not exist (see for instance Taylor et al, 2011; Antonescu et al, 2011; reviewed in Kirchhausen et al, 2014). Cargo selection in particular is a critical aspect of the formation of a mature and stable CCP, and many of the proteins involved in the initiation and maturation of a CCP contribute to cargo selection and are themselves stabilized upon incorporation of cargo into the nascent vesicle (reviewed in Kirchhausen et al, 2014; McMahon and Boucrot, 2011).
Although the clathrin triskelion was identified early as a major component of the coated vesicles, clathrin does not bind directly to membranes or to the endocytosed cargo. Vesicle formation instead relies on many proteins and adaptors that can bind the plasma membrane and interact with cargo molecules. Cargo selection depends on the recognition of endocytic signals in cytoplasmic tails of the cargo proteins by adaptors that interact with components of the vesicle's inner coat. The classic adaptor for clathrin-coated vesicles is the tetrameric AP-2 complex, which along with clathrin was identified early as a major component of the coat. Some cargo indeed bind directly to AP-2, but subsequent work has revealed a large family of proteins collectively known as CLASPs (clathrin- associated sorting proteins) that mediate the recruitment of diverse cargo into the emerging clathrin-coated vesicles (reviewed in Traub and Bonifacino, 2013). Many of these CLASP proteins themselves interact with AP-2 and clathrin, coordinating cargo recruitment with coat formation (Schmid et al, 2006; Edeling et al, 2006; reviewed in Traub and Bonifacino, 2013; Kirchhausen et al, 2014).
Initiation of CCP formation is also influenced by lipid composition, regulated by clathrin-associated phosphatases and kinases (reviewed in Picas et al, 2016). The plasma membrane is enriched in PI(4,5)P2. Many of the proteins involved in initiating clathrin-coated pit formation bind to PI(4,5)P2 and induce membrane curvature through their BAR domains (reviewed in McMahon and Boucrot, 2011; Daumke et al, 2014). Epsin also contributes to early membrane curvature through its Epsin N-terminal homology (ENTH) domain, which promotes membrane curvature by inserting into the lipid bilayer (Ford et al, 2002).
Following initiation, some CCPs progress to formation of vesicles, while others undergo disassembly at the cell surface without producing vesicles (Ehrlich et al, 2004; Loerke et al, 2009; Loerke et al, 2011; Aguet et al, 2013; Taylor et al, 2011). The assembly and stabilization of nascent CCPs is regulated by several proteins and lipids (Mettlen et al, 2009; Antonescu et al, 2011).
Maturation of the emerging clathrin-coated vesicle is accompanied by further changes in the lipid composition of the membrane and increased membrane curvature, promoted by the recruitment of N-BAR domain containing proteins (reviewed in Daumke et al, 2014; Ferguson and De Camilli, 2012; Picas et al, 2016). Some N-BAR domain containing proteins also contribute to the recruitment of the large GTPase dynamin, which is responsible for scission of the mature vesicle from the plasma membrane (Koh et al, 2007; Lundmark and Carlsson, 2003; Soulet et al, 2005; David et al, 1996; Owen et al, 1998; Shupliakov et al, 1997; Taylor et al, 2011; Ferguson et al, 2009; Aguet et al, 2013; Posor et al, 2013; Chappie et al, 2010; Shnyrova et al, 2013; reviewed in Mettlen et al, 2009; Daumke et al, 2014). After vesicle scission, the clathrin coat is dissociated from the new vesicle by the ATPase HSPA8 (also known as HSC70) and its DNAJ cofactor auxilin, priming the vesicle for fusion with a subsequent endocytic compartment and releasing clathrin for reuse (reviewed in McMahon and Boucrot, 2011; Sousa and Laufer, 2015). View original pathway at Reactome.
Clathrin-mediated endocytosis involves initiation of clathrin-coated pit (CCP) formation, cargo selection, coat assembly and stabilization, membrane scission and vesicle uncoating. Although for simplicity in this pathway, the steps leading to a mature CCP are represented in a linear and temporally distinct fashion, the formation of a clathrin-coated vesicle is a highly heterogeneous process and clear temporal boundaries between these processes may not exist (see for instance Taylor et al, 2011; Antonescu et al, 2011; reviewed in Kirchhausen et al, 2014). Cargo selection in particular is a critical aspect of the formation of a mature and stable CCP, and many of the proteins involved in the initiation and maturation of a CCP contribute to cargo selection and are themselves stabilized upon incorporation of cargo into the nascent vesicle (reviewed in Kirchhausen et al, 2014; McMahon and Boucrot, 2011).
Although the clathrin triskelion was identified early as a major component of the coated vesicles, clathrin does not bind directly to membranes or to the endocytosed cargo. Vesicle formation instead relies on many proteins and adaptors that can bind the plasma membrane and interact with cargo molecules. Cargo selection depends on the recognition of endocytic signals in cytoplasmic tails of the cargo proteins by adaptors that interact with components of the vesicle's inner coat. The classic adaptor for clathrin-coated vesicles is the tetrameric AP-2 complex, which along with clathrin was identified early as a major component of the coat. Some cargo indeed bind directly to AP-2, but subsequent work has revealed a large family of proteins collectively known as CLASPs (clathrin- associated sorting proteins) that mediate the recruitment of diverse cargo into the emerging clathrin-coated vesicles (reviewed in Traub and Bonifacino, 2013). Many of these CLASP proteins themselves interact with AP-2 and clathrin, coordinating cargo recruitment with coat formation (Schmid et al, 2006; Edeling et al, 2006; reviewed in Traub and Bonifacino, 2013; Kirchhausen et al, 2014).
Initiation of CCP formation is also influenced by lipid composition, regulated by clathrin-associated phosphatases and kinases (reviewed in Picas et al, 2016). The plasma membrane is enriched in PI(4,5)P2. Many of the proteins involved in initiating clathrin-coated pit formation bind to PI(4,5)P2 and induce membrane curvature through their BAR domains (reviewed in McMahon and Boucrot, 2011; Daumke et al, 2014). Epsin also contributes to early membrane curvature through its Epsin N-terminal homology (ENTH) domain, which promotes membrane curvature by inserting into the lipid bilayer (Ford et al, 2002).
Following initiation, some CCPs progress to formation of vesicles, while others undergo disassembly at the cell surface without producing vesicles (Ehrlich et al, 2004; Loerke et al, 2009; Loerke et al, 2011; Aguet et al, 2013; Taylor et al, 2011). The assembly and stabilization of nascent CCPs is regulated by several proteins and lipids (Mettlen et al, 2009; Antonescu et al, 2011).
Maturation of the emerging clathrin-coated vesicle is accompanied by further changes in the lipid composition of the membrane and increased membrane curvature, promoted by the recruitment of N-BAR domain containing proteins (reviewed in Daumke et al, 2014; Ferguson and De Camilli, 2012; Picas et al, 2016). Some N-BAR domain containing proteins also contribute to the recruitment of the large GTPase dynamin, which is responsible for scission of the mature vesicle from the plasma membrane (Koh et al, 2007; Lundmark and Carlsson, 2003; Soulet et al, 2005; David et al, 1996; Owen et al, 1998; Shupliakov et al, 1997; Taylor et al, 2011; Ferguson et al, 2009; Aguet et al, 2013; Posor et al, 2013; Chappie et al, 2010; Shnyrova et al, 2013; reviewed in Mettlen et al, 2009; Daumke et al, 2014). After vesicle scission, the clathrin coat is dissociated from the new vesicle by the ATPase HSPA8 (also known as HSC70) and its DNAJ cofactor auxilin, priming the vesicle for fusion with a subsequent endocytic compartment and releasing clathrin for reuse (reviewed in McMahon and Boucrot, 2011; Sousa and Laufer, 2015). View original pathway at Reactome.
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for clathrin-mediated
endocytosisA number of studies have been performed to test whether AP-2 is essential for all forms of clathrin-mediated endocytosis (Keyel et al, 2006; Motely et al, 2003; Huang et al, 2004; Boucrot et al, 2010; Henne et al, 2010; Johannessen et al, 2006; Gu et al, 2013; reviewed in Traub, 2009; McMahon and Boucrot, 2011). Although depletion of AP-2 differentially affects the endocytosis of different cargo, extensive depletion of AP-2 through RNAi reduces clathrin-coated pit formation by 80-90%, and the CCPs that do form still contain AP-2, highlighting the critcical role of this complex in CME (Johannessen et al, 2006; Boucrot et al, 2010; Henne et al, 2010).
In addition to AP-2, a wide range of other CLASPs including proteins of the beta-arrestin, stonin and epsin families, engage sorting motifs in other cargo and interact either with clathrin, AP-2 or each other to facilitate assembly of a clathin-coated pit (reviewed in Traub and Bonifacino, 2013).
Annotated Interactions
AP-2 recruitment is also promoted by conformational changes upon lipid and protein binding. AP-2 is a heterotetramer consisting of two large subunits (alpha and beta1 adaptin), a medium mu2 subunit and a small sigma2 subunit, and exists in a closed conformation when not part of a clathrin-coated pit (Jackson et al, 2010).
Interactions between the AP-2 mu2 subunit and PIP2 within the lipid bilayer stabilize the 'open' conformation of AP-2, exposing binding sites for cargo proteins. The open conformation is also promoted by interaction of AP-2 with early CCP proteins such as SGIP and FCHo2 (Hollopeter et al, 2014). Recruitment of clathrin stimulates the activity of AAK1, an AP-2 kinase that phosphorylates the mu2 subunit of the adaptor complex at Thr156, further stabilizing the open conformation and promoting cargo recruitment (Olusanya et al, 2001; Ricotta et al, 2002; Conner et al, 2002; Conner et al, 2003).
NECAP1 and 2 may also aid in the assembly of an emergent clathrin-coated pit. NECAP proteins have a WxxF motif at the C-terminus that binds with high affinity to the alpha-ear sandwich domain of AP-2 and an N-terminal PH ear domain that interacts both with AP-2 and a wide range of endocytic accessory proteins containing FxDxF motifs (Ritter et al, 2003; Wasiak et al, 2002; Ritter et al, 2013). Clathrin and the NECAP PH ear domain appear to compete for an AP-2 binding site. Clathrin-mediated displacement of the NECAP PH ear domain from its lower affinity AP-2 site may allow release this domain, allowing it to transition to a role in recruiting endocytic accessory proteins and cargo (Ritter et al, 2007; Ritter et al, 2013; reviewed in McMahon and Boucrot, 2011).
Finally, studies have highlighted a role for ARF6 and its GTPase activating protein ARFGAP1 in CCP formation, although the details remain to be established.
ARFGAP1 and ARF6 appear to contribute to the recruitment of some cargo, but may also play a more generalized role in CCP formation (Moravec et al, 2012; Bai et al, 2011). ARFGAP1 binds directly to AP-2 and its GAP activity is required for CME. Consistent with this, silencing of ARFGAP1 impairs CME (Schmid et al, 2006; Rawet et al 2010; Bai et al 2011). ARFGAP1 has activity towards several ARFs, including ARF6 which is found is some CCPs and is known to regulate CME under some circumstances (Moravec et al, 2003; Palacios et al, 2002; Paleotti et al, 2005; Kraus et al, 2003). ARF6 is thought to contribute to the recruitment of AP-2 and clathrin to the plasma membrane, possibly in part by affecting the lipid composition (Paleotti et al, 2002; Krauss et al, 2003).
FCHo proteins interact with the plasma membrane-enriched PI(4,5)P2 through the F-BAR domain, which recognizes curvature in the membrane (Henne et al, 2010; Henne et al, 2007; Shimada et al, 2007; Umasankar et al, 2012). Other F-BAR proteins, such as FNBP1 and FNBP1L may join the nascent clathrin-coated pit at a slightly later stage (Shimada et al, 2007). Recruitment of EPS15 and ITSN1 and 2 appears coincident with binding of FCHo2 and depends on direct interaction with the AP2 mu homology domain of FCHo2 (Henne et al, 2010).
SGIP1 (Src homology 3-domain growth factor receptor-bound 2-like (endophilin) interacting protein 1) interacts with numerous endocytic proteins including AP-2, ITSN1, REPS1, EPS15, endophilin and amphiphsyin1 and is thought to play a role in clathrin-mediated endocytosis (Trevaskis et al, 2005; Dergai et al, 2010; Uezu et al, 2007). SGIP1 is related to the FCHo proteins and is co-immunoprecipitated in a tripartite complex containing ITSN1 and REPS1 (Dergai et al, 2010). The exact function of SGIP1 in clathrin-mediated endocytosis remains to be elucidated, however recent work suggests SGIP1 and FCHo proteins may contribute to allosteric changes in AP-2 that promote membrane binding and cargo recognition (Hollopeter et al, 2014).
The recruitment of this group of early CCP proteins is rapidly followed by the incorporation of many AP-2 and clathrin molecules, stimulated in part by the FCHo- and SGIP-dependent stabilization of the open, membrane binding conformation of AP-2 (Hollopeter et al, 2014). Alternately, a proportion of the nascent CCPs may undergo abortive initiation (Loerke et al, 2009; Aguet et al, 2013; Antonescu et al, 2011). This is prompted in part through the early recruitment of the 170 kDa isoform of synaptojanin 1 (SYNJ1-170, not shown in this reaction). SYNJ1 catalyzes the hydrolysis of PI(4,5)P2 to PI(4)P and destabilizes the interaction of many early CCP components with the plasma membrane (Perera et al, 2006).
HIP1 and HIP1R are additional components of the late clathrin-coated pit that interact with clathrin and AP-2 and may contribute to actin nucleation (Waelter et al, 2001; Mishra et al, 2001;Metzler et al, 2001; Legendre-Guillemin et al, 2002; Wilbur et al, 2008; Taylor et al, 2011).
In addition to SYNJ1 and 2, other inositol-5-phosphatases are also recruited to the CCP at the time of scission. These include OCRL, which is recruited through interaction with clathrin as well as the RAB5 interactor APPL1 (Erdmann et al, 2007; Mao et al, 2009; Taylor et al, 2011; Nandez et al, 2014).
CCP formation is a highly heterogeneous and dynamic process and includes abortive initiation of nearly half of nascent CCPs (Loerke et al, 2009; Aguet et al, 2013). Heterogeneity is in part the result of the widely varied cargo proteins, which compete for a limited number of interaction hubs on AP-2 and clatrhin and influence the other protein components of the CCPs. Heterogeneity may also be partly stochastic, or be influenced by the presence of CCP 'hot spots' in the plasma membrane (Taylor et al, 2011; Antonescu et al, 2011; Gaidarov et al, 1999; Ehrlich et al, 2004; Saffarian et al, 2009; Nunez et al, 2011). It is important to note that although events in this pathway are depicted as occuring sequentially in a defined order, in reality the assembly of a clathrin-coated vesicle may be highly variable and the temporal boundaries are likely less clearly defined. Moreover, not every CCP will have all of the proteins indicated in this pathway.