The Fc gamma receptors (FCGRs) have been reported to facilitate Leishmania internalization, especially when in its amastigote form (Ueno et al. 2012). Following cell-to-cell propagation within an established infection or reinfection of a previously infected host, the IgG produced by the host covers the surface of Leishmania amastigote parasites, making them more susceptible to phagocytosis through FCGRs (Polando et al. 2013).
Classically, phagocytosis via FCGRs has been associated with the subsequent activation of Rac GTPases and Cdc42 which in turn activate the phagocyte's NADPH oxidase, contributing to the activation of killing mechanisms (Ueno et al. 2012).
View original pathway at Reactome.
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In view of the highly variable nature of antibody proteins, this biological object is an approximate and fragmented representation of an IgM/IgD antibody, given the limitations of Ig chain enumeration in UniProt. A single mRNA transcript is alternatively spliced to give either IgM or IgD. Thus unactivated B cells contain both classes of antibody.
Myosin-X (Myosin 10) is one of the downstream effectors of PI3K in FCGR-phagocytosis and is involved in pseudopod extension and closure of phagocytic cups. It is recruited to the forming phagosome by binding, through its second PH domain to membrane PIP3, a major product of PI3-kinase (Cox et al. 2002). Myosin-X may act as a motor to transport membrane cargo molecules to the forming pseudopods, influencing actin dynamics. It is not understood with certainty how myosin X contributes to the mechanism of pseudopod extension. It selectively binds to actin bundle such that each head may bind, in an ATP-sensitive manner, to two adjacent actin filaments within the actin bundle. Myosin X hydrolyze ATP and converts this chemical energy to mechanical energy moving toward the plus end/barbed end of the actin filament facing towards the tip of the growing pseudopods (Araki 2006, Chavrier 2003, Watanabe et al 2010).
FCGR mediated phagocytosis requires CDC42 to stimulate actin polymerization, generating the force for phagocytic cup protrusion or pseudopod extension. CDC42 activation is restricted at the advancing edge of the phagocytic cup, where actin is concentrated, and is deactivated at the base of the phagocytic cup (Beemiller et al 2010). The mechanism behind the recruitment and activation of CDC42 during FCGR phagocytosis is unknown. VAV regulates the activation of RAC1 but not CDC42 and the GEF responsible for CDC42 activation during FCGR-mediated phagocytosis remains unidentified (Adam et al 2004, Patel et al 2002).
WASP family verprolin-homologous proteins (WAVEs) function downstream of RAC1 and are involved in activation of the ARP2/3 complex. The resulting actin polymerization mediates the projection of the plasma membrane in lamellipodia and membrane ruffles. WAVEs exist as a pentameric hetero-complex called WAVE Regulatory Complex (WRC). The WRC consists of a WAVE family protein (WASF1, WASF2 or WASF3 - commonly known as WAVE1, WAVE2 or WAVE3), ABI (Abelson-interacting protein), NCKAP1 (NAP1, p125NAP1), CYFIP1 (SRA1) or the closely related CYFIP2 (PIR121), and BRK1 (HSPC300, BRICK). Of the three structurally conserved WAVEs in mammals, the importance of WAVE2 in activation of the ARP2/3 complex and the consequent formation of branched actin filaments is best established. WAVEs in the WRC are intrinsically inactive and are stimulated by RAC1 GTPase and phosphatidylinositols (PIP3). The C-terminal VCA domain of WAVE2 (and likely WAVE1 and WAVE3) which can bind both the ARP2/3 complex and actin monomers (G-actin) is masked in the inactive state. After PIP3 binds to the polybasic region of WAVE2 (and likely WAVE1 and WAVE3) and RAC1:GTP binds to the CYFIP1 (or CYFIP2) subunit of the WRC, allosteric changes most likely occur which allow WAVEs to interact with the ARP2/3 complex. The interactions between WAVEs and RAC1 are indirect. BAIAP2/IRSp53, an insulin receptor substrate, acts as a linker, binding both activated RAC1 and the proline-rich region of WAVE2 (and likely WAVE1 and WAVE3) and forming a trimolecular complex. CYFIP1 (or CYFIP2) in the WAVE regulatory complex binds directly to RAC1:GTP and links it to WAVE2 (and likely WAVE1 and WAVE3) (Derivery et al. 2009, Yamazaki et al. 2006, Takenawa & Suetsugu 2007, Chen et al. 2010, Pollard 2007, Lebensohn & Kirschner 2009).
Once activated, the ARP2/3 complex nucleates new actin filaments that extend from the sides of pre-existing mother actin filaments at a 70-degree angle to form Y-branched networks (Firat-Karalar & Welch 2010). These branched actin filaments push the cell membrane forward to form a pseudopod. The ARP2/3 complex is composed of two Arps (actin-related proteins), ARP2 and ARP3, and five unique proteins ARPC1, ARPC2, ARPC3, ARPC4 and ARPC5 (Gournier et al. 2001). Both ARP2 and ARP3 subunits bind ATP. There are two proposed models to explain the process of actin nucleation by ARP2/3 complex: the barbed-end branching model and the dendritic nucleation/side branching model (Le Clainche & Carlier 2008). In barbed-end branching model, the branching/ternary complex (G-actin-WASP/WAVE-Arp2/3 complex) binds to the barbed end of the mother filament. G-actin bound to VCA domain or one of the Arp subunits incorporates into the mother filament at the barbed end, thus positioning ARP2/3 complex to initiate the daughter branch on the side of the mother filament. ARP2/3 nucleates the formation of new actin filament branches, which elongate at the barbed ends (Le Clainche & Carlier 2008, Pantaloni et al 2000, Le Clainche et al. 2003, Egile et al. 2005). In side branching model, the branching complex binds to the side of the mother actin filament mimicking an actin nucleus and initiates a lateral branch (Le Clainche & Carlier 2008, Amann & Pollard 2001).
The ARP2/3 complex shows higher affinity for the phosphorylated VCA domain of WAVE2 than for the unphosphorylated VCA domain. WAVE proteins can be phosphorylated by various kinases. Active ERK (Mitogen activated protein kinase 3) phosphorylates the WAVE regulatory complex (WRC) on multiple serine/threonine sites within the proline-rich domains (PRDs) of WAVE2 and ABI1. Phosphorylation of the PRDs would disrupt their interaction with SH3 and PLP binding domains, potentially altering WRC activation. ERK phosphorylates both S343 and T346 in WAVE2 and S183, S216, S225, S392, and S410 in ABI1. Cumulatively, the phosphorylation of both WAVE2 and ABI in the WAVE regulatory complex (WRC) contributes to the RAC-induced WRC conformational change that exposes the VCA domain, leading to binding and activation of ARP2/3 (Mendoza et al. 2011, Nakanishi et al. 2007). ERK phosphorylation sites in WAVE2 are not strictly conserved in WAVE1 and WAVE3 but, based on the amino acid sequence, other potential ERK phosphorylation sites exist.
ATP bound G-actin monomers are added to the fast growing barbed ends of both mother and daughter filaments. The polymerization of these filaments drives membrane protrusion. In the process of phagocytosis, pseudopodia extend around the antibody-bound particle to form the phagocytic cup. This elongation continues until the filament reaches steady state equilibrium with free G-actin monomers (Millard et al. 2004, Le Clainche et al. 2008).
Abelson interactor-1 (ABL) tyrosine kinase phosphorylates the strictly conserved tyrosine 150 in WAVE2 (Y151 in WAVE1 and WAVE3) (Leng et al. 2003, Chen et al. 2010).
After incorporation at the branch, the actin bound to VCA domain of WASP/WAVE undergoes ATP hydrolysis and this destabilizes its interaction with WASP/WAVE. This dissociates the branched junction from the membrane-bound WASP/WAVE (Kovar 2006).
WASP interacting proteins (WIP) family includes WIPF1 (WIP), WIPF2 (WIRE,WICH) and WIPF3 (CR16, corticosteroids and regional expression-16). WIPs share a specific proline rich sequence that interacts with the WH1 domain of WASP and N-WASP (WASL). WIPs form heterocomplexes with WASPs and may contribute to the WASP protein stability (Aspenstrom 2002, Kato et al. 2002, Ho et al. 2001, Moreau et al. 2000). SH3 domain containing adaptor proteins like GRB2 (Carlier et al. 2000), NCK (Rohatgi et al. 2001) and WISH (DIP/SPIN90) (Fukuoka et al. 2001) bind to the proline rich domain in WASPs and activate the ARP2/3 complex. By binding simultaneously to N-WASP and the ARP2/3 complex, GRB2 works synergistically with CDC42 in the activation of ARP2/3 complex-mediated actin assembly (Carlier et al. 2000).
WASP is phosphorylated on Tyr291 (Cory et al. 2002) and N-WASP (WASL) on Tyr256 (Wu et al. 2004) by Src family of tyrosine kinases and this phosphorylation may release the autoinhibitory intramolecular interactions. The phosphorylation seems to be enhanced by the activation of CDC42. WASP phosphorylation and binding of CDC42 have a synergistic effect on the activation of the ARP2/3 complex (Takenawa & Suetsugu 2007). In N-WASP, the phosphorylation may reduce its nuclear translocation and may sustain it in its functional site in the cytoplasm (Wu et al. 2004).
Vav interacts directly with PIP2 and PIP3, with a fivefold selectivity for PIP3 over PIP2. PIP3 gives a twofold stimulation of Vav1 GEF activity while PIP2 leads to 90% inhibition. Binding probably occurs through the PH domain, known to bind phosphoinositides.
Wiskott-Aldrich syndrome protein (WASP) and Neural-WASP (N-WASP, WASL) proteins are scaffolds that transduce signals from cell surface receptors to the activation of the ARP2/3 complex and actin polymerization. WASP and N-WASP possess a central GTPase binding domain (GBD) and an NH2-terminal WASP homology domain 1 (WH1) followed by a basic region (B), and a C-terminal VCA region that contains: a V domain (verprolin homology/WASP homology 2), a C domain (connecting), and an A motif (acidic). The VCA region is responsible for binding to and activating the ARP2/3 complex (Bompard & Caron 2004, Callebaut et al 1998). Under resting conditions, WASP and N-WASP are maintained in an autoinhibited state via interaction of the GBD and the VCA domains. This prevents access of the ARP2/3 complex and G-actin to the VCA region. Activated CDC42 binds to the GBD region of WASPs and this interaction releases the VCA region from autoinhibition, enabling binding of the ARP2/3 complex and stimulating actin polymerization (Kim et al 2000, Park & Cox 2009). Phosphoinositides (PtdIns(4,5)P2) interact with the basic (B) region in WASPs and this interaction is important for activation of the WASPs and the ARP2/3 complex (Higgs & Pollard 2000).
Once WASPs (WASP and N-WASP) and WAVEs (WAVE2 and probably WAVE1 and WAVE3) are activated, their VCA region becomes available for binding to the ARP2/3 complex and actin monomer (G-actin). The actin monomer binds to the V domain and ARP2/3 complex binds to the CA domain. The simultaneous binding of G-actin and the ARP2/3 complex to the VCA region contributes to the activation of the ARP2/3-complex-mediated actin polymerization. The VCA module acts as a platform on which an actin monomer binds to the ARP2/3 complex to trigger actin polymerization (Takenawa & Suetsugu 2007).
Multiple sites of phosphorylation are known to exist in SYK, which both regulate its activity and also serve as docking sites for other proteins. Some of these sites include Y131 of interdomain A, Y323, Y348, and Y352 of interdomain B, and Y525 and Y526 within the activation loop of the kinase domain and Y630 in the C-terminus (Zhang et al. 2002, Lupher et al. 1998, Furlong et al. 1997). Phosphorylation of these tyrosine residues disrupts autoinhibitory interactions and results in kinase activation even in the absence of phosphorylated ITAM tyrosines (Tsang et al. 2008). SYK is primarily phosphorylated by Src family kinases and this acts as an initiating trigger by generating few molecules of activated SYK which are then involved in major SYK autophosphorylation (Hillal et al. 1997).
SYK is a tyrosine kinase related to ZAP70 that is expressed in all hematopoietic cells and coimmunoprecipitates with the gamma chain associated with FCGRIIIA in macrophages and with FCERI in mast cells. SYK is very important for FCGR phagocytosis and is recruited to these phosphorylated ITAM residues through its two SRC homology 2 (SH2) domains (Agarwal et al. 1993). When SYK kinase expression is inhibited with antisense oligonucleotides both in vitro and in vivo, phagocytosis and inflammation are abolished (Matsuda et al. 1997). The domain structure of SYK comprises a regulatory region at the N-terminus consisting of a pair of SH2 domains separated by an inter-SH2 linker called interdomain A, an SH2-domain-kinase linker termed interdomain B, and a C-terminal kinase domain (Arias-Palomo et al. 2009). In resting state SYK exists in an auto-inhibited conformation by the interactions between the SH2-SH2 regulatory region and the inter-SH2 linker and the catalytic domain. This interdomain interaction reduces the conformational flexibility required by the kinase domain for catalysis (Arias-Palomo et al. 2007). Changes in the orientation of the SH2 domains could control the disruption of the auto inhibitory interactions and the activation of SYK. These movements could be totally or partially induced by the binding to phosphorylated ITAMs and/or phosphorylation of tyrosine residues in interdomain A or B (Arias-Palomo et al. 2009). Tsang et al. suggested that SYK functions as an OR-gate switch with respect to phosphorylation and ITAM binding, as either one stimulus OR the other is sufficient to cause full activation (Tsang et al. 2008).
After cross linking, Fc gamma receptors are sequestered to lipid rafts where they are complexed with some of the tyrosine kinases of Src family and undergo phosphorylation on the tyrosine residues contained in conserved ITAM sequences. At least six out of nine members of the Src family kinases (SRC, FYN, FGR, HCK, YES and LYN ) have been identified in the phagocytic cells and are implicated in the initiation of Fc gamma mediated signaling. (Suzuki et al. 2000, Majeed et al. 2001, Kwiatkowska et al. 2003). Some of these kinases have been found associated with specific receptors. In monocytes HCK and LYN have been found associated with FCGRI (Durden et al. 1995), whereas only HCK with FCGRIIA (Ghazizadeh et al. 1994) while FGR in neutrophils (Hamada et al. 1993) and LCK in NK cells with FCGRIIIA (Pignata et al. 1993) The implication of Src kinases in phosphorylation was first supported by pharmacological findings that herbimycin A, a tyrosine kinase inhibitor relatively specific for Src-family kinases, potently suppressed Fc receptor mediated functions (Greenberg et al. 1993, Suzuki et al. 2000). However, their particular involvement in phagocytosis remains unclear, as targeted disruption of single or multiple Src family genes did not result in significant alterations in phagocytosis (Hunter et al. 1993, Fitzer Attas et al. 2000, Suzuki et al. 2000). HCK, FGR and LYN triple-deficient (-/-) macrophages have shown significant delays in FCGR mediated phagocytosis, but these deficiencies do not completly disrupt the process (Fitzer Attas et al. 2000). Tyrosine residues Y288 and Y304 (Y282 and Y298 according to the literature reference, it is 6 residues shorter compared to uniprot entry due to an alternate initiation codon usage), within ITAM sequence in the cytoplasmic domain of FCGRIIA are the key target sites that are phosphorylated by Src family kinases (Mitchell et al, 1994). In case of FCGRIA and FCGRIIIA the specific tyrosine residues within ITAMs of the associated gamma/zeta chains are phosphorylated by activated Src family kinases (SFKs) (Park et al. 1993).
The internalization of Leishmania amastigotes by macrophages is thought to be mediated mainly through opsonization with immunoglobulins (Igs) which bind FcγRs, stimulating the uptake (Morehead et al 2002 & Padigel et al. 2005). Glycoinositol phospholipids (GIPLs) are the most abundant glycolipids on the surface of the amastigote form of Leishmania parasites and Buxbaum and colleagues showed that IgG1 in mice, binds the GIPL molecules on the amastigote stage of L. mexicana to subsequently induced the phagocytosis through FcγRs (Buxbaum 2013).
FCGRIII (CD16) is a low affinity Fc gamma receptor and is encoded by two genes (A and B), the transmembrane form FCGRIIIA and the GPI anchored FCGRIIIB (Edberg et al. 1989). FCGRIIIA is involved in phagocytosis and is expressed in macrophages and natural killer cells as a multi chain complex consisting of a single alpha chain containing IgG binding domains and a signal transducing gamma and/or zeta dimer (Wirthmuller et al. 1992, Lanier et al. 1989, Garcia Garcia & Rosales 2002). Both gamma and zeta chains contain a conserved immunoreceptor tyrosine based activation motif (ITAM), which has 2 copies of the YXXL sequence (Isakov 1997). However, the gamma chain of FCGRIIIA is approximately sixfold more efficient in mediating phagocytosis than the zeta subunit (Park & Schreiber 1995). Phosphorylation of the conserved tyrosine residues of the ITAM in these accessory proteins is required for the phagocytic signal mediated by FCRGIIIA. The first step in Fc-gamma receptor (FCGR) phagocytosis is binding and clustering of FCGRs by IgG-coated foreign particles (For this particular pathway the coated foreing particle is the Leishmania parasite). FCGR are clustered at the cell surface by multivalent antigen-antibody complexes and recruited to lipid raft micro domains; monovalent ligand binding is insufficient to generate a signal. This cross linking results in the localisation of FCGRs into lipid rafts and this may aid in their recruiting and complexing with additional signalling proteins associated with lipid rafts (Kono et al. 2002). This is followed by phosphorylation of the tyrosine residues within the ITAM located on the cytoplasmic portion of accessory gamma/zeta chains by membrane associated tyrosine kinases of the Src family (Park et al. 1993).
VAV proteins exist in an auto-inhibitory state folded in such a way as to inhibit the GEF activity of its DH domain. This folding is mediated through binding of tyrosines in the acidic domain to the DH domain and through binding of the CH domain to the C1 region. Activation of VAV may involve at least three different events to relieve this auto-inhibition. Phosphorylation of the tyrosines in the acidic domain causes them to be displaced from the DH domain, binding of a ligand to the CH domain may cause it to release the C1 domain and binding of PIP3 to PH domain may alter its conformation. VAV1 is phosphorylated on Y174 in the acidic domain, and this is mediated by Syk and Src-family tyrosine kinases. Once activated, VAV1 is then involved in the activation of RAC and CDC42 downstream of FCGR.
Macrophages lacking all the three isoforms of VAV did not affect FCGR-mediated phagocytosis suggesting that RAC1 is regulated by GEFs other than VAV downstream of the FCGR (Hall et al 2006). DOCK180, a member of GEFs, is found to be involved in the activation of RAC1. DOCK180 associates with the adaptor protein CRKII and the complex is found to accumulate at the phagocytic cup. DOCK180 is recruited to the sites of phagocytosis by binding to SH3 domain of CRKII through its proline-rich motif (Hasegawa et al 1996). CRKII is likely recruited to the activated FCGR complex by binding phosphorylated ITAM tyrosines on the receptor or through other phosphotyrosines on ancillary proteins that are recruited to the receptor complex (Lee et al 2007). Unlike the usual GEFs, DOCK180 does not contain the conserved Dbl homology (DH) domain. Instead, it has a DHR-2 or DOCKER domain capable of loading RAC with GTP (Brugnera et al 2002). Binding of DOCK180 to RAC alone is insufficient for GTP loading, and a DOCK180-ELMO interaction is required. ELMO1, as well as ELMO2, form a complex with DOCK180 and they function together as a bipartite GEF to optimally activate RAC (Gumienny et al 2001, Brugnera et al 2002).
The organized movements of membranes and the actin cytoskeleton are coordinated in phagocytosis by small GTPases of the Rho family. Specifically, RAC1 and CDC42 are known to be stimulated upon engagement of FCGR and are essential for the extension of the pseudopods that surround and engulf the phagocytic particle (Scott et al 2005). RAC1 is known to regulate actin dynamics. It is active throughout the phagocytic cup and activated RAC1 is necessary to assemble F actin. However, closing the phagocytic cup requires RAC1 to be deactivated (Naakaya et al 2007). Deletion of RAC1 prevents FCGR mediated phagocytosis (Hall et al 2006). RAC1 activation involves transition from an inactive GDP bound to an active GTP bound state catalysed by guanine exchanges factors (GEFs). VAV has been implicated in the activation of RAC1 (Patel et al 2002).
The internalization of Leishmania amastigotes by macrophages is thought to be mediated mainly through opsonization with immunoglobulins (Igs) which bind Fc gamma receptors (FCGRs), stimulating their uptake (Morehead et al 2002 & Padigel et al. 2005). Glycoinositol phospholipids (GIPLs) are the most abundant glycolipids on the surface of the amastigote form of Leishmania parasites and Buxbaum and colleagues showed that IgG1 in mice binds GIPL molecules on the amastigote stage of L. mexicana to subsequently induce phagocytosis through FCGRs (Buxbaum 2013).
VAV family members are cytoplasmic guanine nucleotide exchange factors (GEFs) for Rho-family GTPases (RAC, RHO and CDC42). VAV1 is found predominantly in hematopoietic cells, whereas VAV2 and VAV3 are more broadly expressed. VAV proteins link the cell surface receptors like FCGR to the intracellular Rho GTPases and the actin cytoskeleton during phagocytosis (Hall et al 2006). Experiments using two-hybrid system suggest that VAV1 with its SH2 domain directly binds to the phosphorylated Y342 of SYK (Deckert et al. 1996). VAV proteins are also recruited to membrane through their PH domain by binding PI(3,4,5)P3 produced by PI3K.
In addition to the membrane remodeling for pseudopod extension, particle internalization requires a contractility force pulling the forming phagosome into the cytoplasm. Myosin motor proteins are the actin-binding proteins, with ATPase activity move along actin fibers, and produce the driving force for phagosome formation and transport. Several myosin motors including myosins IC, II, V, IXb are involved in FCGR-mediated phagocytosis as force generators and actin-based transport motors (Swanson et al. 1999). Nonmuscle myosin II, is a motor protein known to generate intracellular contractile forces and tension by associating with F-actin. It has been observed to localize around forming phagosomes and suggested a role in phagocytic-cup squeezing during FCGR-mediated phagocytosis. Each myosin II motor protein exists as a complex consisting of two copies each of myosin II heavy chain (MHC), essential light chains (ELC), and myosin regulatory light chain (MRLC). Selective inhibition of myosin II by ML-7, a myosin light-chain kinase (MLCK) inhibitor, prevents phagocytic cup closure, but not pseudopod extension for the formation of phagocytic cups in FCGR-mediated phagocytosis (Grooves et al. 2008, Araki 2006). Tight ring of actin filaments within the elongating pseudopodia squeezes the deformable particles. In the classical zipper model for phagocytosis, the pseudopod extends over the IgG-coated particles, in which FCGRs in the phagocyte plasma membrane interact sequentially with Fc portions of IgG molecules zippering the membrane along the particle. This sequential IgG-FCGR binding might not occur by itself, but requires forced zipper closure, where myosin-II contractile activity may promote the binding between the FCGR and its ligands, to facilitate the efficient extension and subsequent closure of phagocytic cups (Araki 2006, ). Myosin IC mediates the purse-string-like contraction that closes phagosomes. Myosin-V has been implicated in membrane trafficking events (Swanson et al. 1999).
Wiskott-Aldrich syndrome protein (WASP) and Neural-WASP (N-WASP, WASL) proteins are scaffolds that transduce signals from cell surface receptors to the activation of the ARP2/3 complex and actin polymerization. WASP and N-WASP possess a central GTPase binding domain (GBD) and an NH2-terminal WASP homology domain 1 (WH1) followed by a basic region (B), and a C-terminal VCA region that contains: a V domain (verprolin homology/WASP homology 2), a C domain (connecting), and an A motif (acidic). The VCA region is responsible for binding to and activating the ARP2/3 complex (Bompard & Caron 2004, Callebaut et al 1998). Under resting conditions, WASP and N-WASP are maintained in an autoinhibited state via interaction of the GBD and the VCA domains. This prevents access of the ARP2/3 complex and G-actin to the VCA region. Activated CDC42 binds to the GBD region of WASPs and this interaction releases the VCA region from autoinhibition, enabling binding of the ARP2/3 complex and stimulating actin polymerization (Kim et al 2000, Park & Cox 2009). Phosphoinositides (PtdIns(4,5)P2) interact with the basic (B) region in WASPs and this interaction is important for activation of the WASPs and the ARP2/3 complex (Higgs & Pollard 2000).
Wiskott-Aldrich syndrome protein (WASP) and Neural-WASP (N-WASP, WASL) proteins are scaffolds that transduce signals from cell surface receptors to the activation of the ARP2/3 complex and actin polymerization. WASP and N-WASP possess a central GTPase binding domain (GBD) and an NH2-terminal WASP homology domain 1 (WH1) followed by a basic region (B), and a C-terminal VCA region that contains: a V domain (verprolin homology/WASP homology 2), a C domain (connecting), and an A motif (acidic). The VCA region is responsible for binding to and activating the ARP2/3 complex (Bompard & Caron 2004, Callebaut et al 1998). Under resting conditions, WASP and N-WASP are maintained in an autoinhibited state via interaction of the GBD and the VCA domains. This prevents access of the ARP2/3 complex and G-actin to the VCA region. Activated CDC42 binds to the GBD region of WASPs and this interaction releases the VCA region from autoinhibition, enabling binding of the ARP2/3 complex and stimulating actin polymerization (Kim et al 2000, Park & Cox 2009). Phosphoinositides (PtdIns(4,5)P2) interact with the basic (B) region in WASPs and this interaction is important for activation of the WASPs and the ARP2/3 complex (Higgs & Pollard 2000).
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surface:FCGR3Ap-CD3
dimers:p-6Y-SYK:VAV1,2,3:PI(3,4,5)P3antigens:FCGR3A:CD3
dimersfilament:branching complex:daughter
filamentfilament:branching
complexN-WASP:ARP2/3
complex:G-actinAnnotated Interactions
surface:FCGR3Ap-CD3
dimers:p-6Y-SYK:VAV1,2,3:PI(3,4,5)P3surface:FCGR3Ap-CD3
dimers:p-6Y-SYK:VAV1,2,3:PI(3,4,5)P3antigens:FCGR3A:CD3
dimersantigens:FCGR3A:CD3
dimersfilament:branching complex:daughter
filamentfilament:branching complex:daughter
filamentfilament:branching complex:daughter
filamentfilament:branching
complexfilament:branching
complexIn barbed-end branching model, the branching/ternary complex (G-actin-WASP/WAVE-Arp2/3 complex) binds to the barbed end of the mother filament. G-actin bound to VCA domain or one of the Arp subunits incorporates into the mother filament at the barbed end, thus positioning ARP2/3 complex to initiate the daughter branch on the side of the mother filament. ARP2/3 nucleates the formation of new actin filament branches, which elongate at the barbed ends (Le Clainche & Carlier 2008, Pantaloni et al 2000, Le Clainche et al. 2003, Egile et al. 2005). In side branching model, the branching complex binds to the side of the mother actin filament mimicking an actin nucleus and initiates a lateral branch (Le Clainche & Carlier 2008, Amann & Pollard 2001).
SH3 domain containing adaptor proteins like GRB2 (Carlier et al. 2000), NCK (Rohatgi et al. 2001) and WISH (DIP/SPIN90) (Fukuoka et al. 2001) bind to the proline rich domain in WASPs and activate the ARP2/3 complex. By binding simultaneously to N-WASP and the ARP2/3 complex, GRB2 works synergistically with CDC42 in the activation of ARP2/3 complex-mediated actin assembly (Carlier et al. 2000).
The implication of Src kinases in phosphorylation was first supported by pharmacological findings that herbimycin A, a tyrosine kinase inhibitor relatively specific for Src-family kinases, potently suppressed Fc receptor mediated functions (Greenberg et al. 1993, Suzuki et al. 2000). However, their particular involvement in phagocytosis remains unclear, as targeted disruption of single or multiple Src family genes did not result in significant alterations in phagocytosis (Hunter et al. 1993, Fitzer Attas et al. 2000, Suzuki et al. 2000). HCK, FGR and LYN triple-deficient (-/-) macrophages have shown significant delays in FCGR mediated phagocytosis, but these deficiencies do not completly disrupt the process (Fitzer Attas et al. 2000).
Tyrosine residues Y288 and Y304 (Y282 and Y298 according to the literature reference, it is 6 residues shorter compared to uniprot entry due to an alternate initiation codon usage), within ITAM sequence in the cytoplasmic domain of FCGRIIA are the key target sites that are phosphorylated by Src family kinases (Mitchell et al, 1994). In case of FCGRIA and FCGRIIIA the specific tyrosine residues within ITAMs of the associated gamma/zeta chains are phosphorylated by activated Src family kinases (SFKs) (Park et al. 1993).
The first step in Fc-gamma receptor (FCGR) phagocytosis is binding and clustering of FCGRs by IgG-coated foreign particles (For this particular pathway the coated foreing particle is the Leishmania parasite). FCGR are clustered at the cell surface by multivalent antigen-antibody complexes and recruited to lipid raft micro domains; monovalent ligand binding is insufficient to generate a signal.
This cross linking results in the localisation of FCGRs into lipid rafts and this may aid in their recruiting and complexing with additional signalling proteins associated with lipid rafts (Kono et al. 2002). This is followed by phosphorylation of the tyrosine residues within the ITAM located on the cytoplasmic portion of accessory gamma/zeta chains by membrane associated tyrosine kinases of the Src family (Park et al. 1993).
N-WASP:ARP2/3
complex:G-actinN-WASP:ARP2/3
complex:G-actin