Argos is a natural secreted antagonist of EGFR signalling which functions by binding growth factor ligands that activate EGFR by forming a clamp like structure using three disulphide-bonded β-sheet domains [].
When the kinase domain of EGFR binds to segment one of Mitogen induced gene 6 (Mig-6), EGFR becomes inactive due to the conformation it adopts which is Src/CDK like. The binding of the two proteins prevents EGFR acting as a cyclin-like activator for other kinase domains [].The structure of Mig-6(1) consists of alpha helices-G and -H with a polar surface and hydrophobic residues for interactions with EGFR. A critical step for the activation of EGFR is the formation of an asymmetric dimer involving the kinase domains of the protein. SinceMig-6 binds to the kinase domain it blocks this process and EGFR becomes inactive [].
Fer () is a tyrosine-protein kinase that acts downstream of cell surface receptors for growth factors and plays a role in the regulation of the actin cytoskeleton, microtubule assembly, lamellipodia formation, cell adhesion, cell migration and chemotaxis [, ]. In non-small cell lung cancer cells, Fer kinase expression has been linked to cell invasion and tumour metastasis []. It also acts downstream of EGFR (EGF receptor) to promote activation of NF-kappa-B and cell proliferation [].
Sorting nexin-5 (SNX5) belongs to the sorting nexin family, which contains a conserved PX (phox homology) domain that is responsible for binding to specific phosphoinositides []. SNX5 also contains a BAR domain that is C terminus to the PX domain. Besides being involved in several stages of intracellular trafficking, SNX5 also plays a role in macropinocytosis and in the internalisation of EGFR after EGF stimulation [, ].
This family consist of zinc finger protein ZPR1 and ZPR1-like proteins. The function of ZPR1 is not clear. ZPR1 is present in both the cytoplasm and nucleoplasma and its subcellular distribution changes during proliferation []. ZPR1 binds to the cytoplasmic tyrosine kinase domain of the epidermal growth factor receptor (EGFR) []and is a component of downstream signaling of both EGFR and FGFR signaling []. It has been associated with susceptibility to type 2 diabetes []and is downregulated in patients with spinal muscular atrophy [].
Leucine-rich repeat flightless-interacting protein 1 (LRRFIP1) is a transcriptional repressor which preferentially binds to the GC-rich consensus sequence (5'-AGCCCCCGGCG-3') and may regulate expression of TNF, EGFR and PDGFA []. It may control smooth muscle cell proliferation following artery injury through PDGFA repression and may also bind double-stranded RNA. It interacts with the leucine-rich repeat domain of human flightless-I (FliI) protein [].Leucine-rich repeat flightless-interacting protein 2 (LRRFIP2) may function as activator of the canonical Wnt signaling pathway, in association with DVL3, upstream of CTNNB1/beta-catenin []. It positively regulates Toll-like receptor (TLR) signalling in response to agonist probably by competing with the negative FLII regulator for MYD88-binding [].
Fer () is a tyrosine-protein kinase that acts downstream of cell surface receptors for growth factors and plays a role in the regulation of the actin cytoskeleton, microtubule assembly, lamellipodia formation, cell adhesion, cell migration and chemotaxis [, ]. In non-small cell lung cancer cells, Fer kinase expression has been linked to cell invasion and tumour metastasis []. It also acts downstream of EGFR (EGF receptor) to promote activation of NF-kappa-B and cell proliferation [].This entry represents the F-BAR domain of Fer. F-BAR domains are dimerization modules that bind and bend membranes and are found in proteins involved in membrane dynamics and actin reorganization [, ].
Leukocyte tyrosine kinase (LTK) receptor is an orphan receptor with a tyrosine-protein kinase activity [, , ]. This receptor is activated by ALKAL1 or ALKAL2 ligands at the cell surface and transduces an extracellular signal into an intracellular response [, ]. Ligand-binding to the extracellular domain induces tyrosine kinase activation, leading to activation of the mitogen-activated protein kinase (MAPK) pathway [].The exact function of this protein is not known. Studies with chimeric proteins (replacing its extracellular region with that of several known growth factor receptors, such as EGFR and CSFIR) demonstrate its ability to promote growth and specifically neurite outgrowth, and cell survival. LTK is involved in regulation of the secretory pathway involving endoplasmic reticulum (ER) export sites (ERESs) and ER to Golgi transport [, , ].
This entry represents the SH3 domain of UBASH3B. UBASH3B (also known as STS-1 or TULA-2), belongs to the TULA family, is an active phosphatase that is expressed ubiquitously []. The phosphatase activity of UBASH3B is essential for its roles in the suppression of TCR signaling []and the regulation of EGFR []. It also interacts with Syk and functions as a negative regulator of platelet glycoprotein VI signaling in platelets []. The TULA family includes two members termed p70/STS-1/TULA-2 and UBASH3A/STS-2/TULA/Cbl-InteractingProtein 4 (CLIP4). TULA proteins contain an N-terminal UBA domain, a central SH3 domain, and a C-terminal histidine phosphatase domain. They bind c-Cbl (a multidomain adaptor and an E3 ubiquitinligase) through the SH3 domain []and to ubiquitin via UBA [].
VAV2 is widely expressed and functions as a guanine nucleotide exchange factor (GEF) for RhoA, RhoB and RhoG and also activates Rac1 and Cdc42 []. It is implicated in many cellular and physiological functions including blood pressure control, eye development, neurite outgrowth and branching, EGFR endocytosis and degradation, and cell cluster morphology, among others [, , , , ]. It has been reported to associate with Nek3. VAV proteins contain several domains that enable their function: N-terminal calponin homology (CH), acidic, RhoGEF (also called Dbl-homologous or DH), Pleckstrin Homology (PH), C1 (zinc finger), SH2, and two SH3 domains. The SH3 domain of VAV is involved in the localization of proteins to specific sites within the cell, by interacting with proline-rich sequences within target proteins [, , ].This entry represents the second SH3 domain of VAV2.
VAV2 is widely expressed and functions as a guanine nucleotide exchange factor (GEF) for RhoA, RhoB and RhoG and also activates Rac1 and Cdc42 []. It is implicated in many cellular and physiological functions including blood pressure control, eye development, neurite outgrowth and branching, EGFR endocytosis and degradation, and cell cluster morphology, among others [, , , , ]. It has been reported to associate with Nek3. VAV proteins contain several domains that enable their function: N-terminal calponin homology (CH), acidic, RhoGEF (also called Dbl-homologous or DH), Pleckstrin Homology (PH), C1 (zinc finger), SH2, and two SH3 domains. The SH3 domain of VAV is involved in the localization of proteins to specific sites within the cell, by interacting with proline-rich sequences within target proteins [, , ].This entry represents the first SH3 domain of VAV2.
This entry represents the zinc finger CCCH-type with G patch domain-containing protein (ZIP) that acts as a transcription repressor through the recruitment of the Mi-2/nucleosome remodelling and deacetylase (NuRD) complex to target promoters. ZIP regulates several cellular signalling pathways like the epidermal growth factor receptor (EGFR) pathways, critically involved in cell proliferation, survival, and migration.ZIP contains several domains: a CCCH zinc finger, a TUDOR domain, a G-patch and a coiled-coil domain. The coiled-coil domain is responsible for the ZIP interaction with Mi-2 to recruit the NuRD complex, while its zinc finger structure specifically recognises the consensus sequence in the 5' upstream region of EGFR. The TUDOR domain is a chromatin-presenting module reading the methylated histone marks. ZIP is also involved in the recognition of RNA and other proteins, suggesting that it may coordinate an active coupling between transcription regulation and pre-mRNA splicing, especially considering that ZIP contains a CCCH-type of zinc finger and a G-patch domain, both of which have been featured in proteins functioning in mRNA processing. Additionally, ZIP inhibits cell proliferation and suppresses breast carcinogenesis, while its depletion leads to a drastic tumour growth in vivo. ZIP is downregulated in breast carcinomas and that its level of expression is negatively correlated with that of EGFR [].
MICAL (molecule Interacting with CasL) family is a group of multifunctional proteins that contain the calponin homology (CH), a LIM and a coiled-coil (CC) domains []. They interact with receptors on the target cells, help recruiting other proteins, and promote the modulation of their activity with respect to the downstream events []. There is only one MICAL protein found in Drosophila [], while there are 5 MICAL (MICAL1/2/3, MICAL-like1/2) isoforms found in vertebrates []. Drosophila MICAL and vertebrate MICAL1/2/3 contain an extra N-terminal FAD (flavin adenine dinucleotide binding monooxygenase) domain, whose structure resembles that of a flavo-enzyme, p-hydroxybenzoate hydroxylase []. Drosophila MICAL has an NADPH-dependent actin depolymerising activity []. Vertebrate MICALs are also shown to be effectors of small Rab GTPases, which play important roles in vesicular trafficking []. MICAL-like protein 1 (MICAL-L1) interacts with small G proteins and regulates endocytic recycling of receptors [, ]. It forms a complex with Rab13 that regulates EGFR trafficking at late endocytic pathways []. MICAL-L1 also forms a complex with Arf6 that regulates Rab8a function. MICAL-L1 can be regulated by Rab35 [].MICAL-like protein 2 (MICAL-L2, also known as JRAB) interacts with Rab13 []and Rab8 to regulate the endocytic recycling of occludin, claudin and E-cadherin to the plasma membrane. It may thereby regulate the establishment of tight junctions and adherens junctions []. MICAL-L2/JRAB also regulates the reorganisation of the actin cytoskeleton through interactions with actinin-1, actinin-4, and filamentous actin [], and via filamins during cell spreading [].
Sorting nexins (SNXs) are a diverse group of cellular trafficking proteins that are unified by the presence of a phospholipid-binding motif, the PX domain. The ability of these proteins to bind specific phospholipids, as well as their propensity to form protein-protein complexes, points to a role for these proteins in membrane trafficking and protein sorting []. Members of this group also contain coiled-coil regions within their large C-terminal domains and a BAR domain, whose function has been defined as a dimerisation motif, as sensing and inducing membrane curvature, and/or likely to bind to small GTPases [].This entry includes SNX5, SNX6 and SNX32 (also known as SNX6B).SNX5 contains a BAR domain that is C teminus to the PX domain. SNX5 plays a role in macropinocytosis []and in the internalisation of EGFR after EGF stimulation [].SNX6 was found to interact with members of the transforming growth factor-beta family of receptor serine/threonine kinases. Strong heteromeric interactions were also seen among SNX1, -2, -4, and -6, suggesting the formation in vivoof oligomeric complexes. SNX6 is localized in the cytoplasm where it is thought to target proteins to the trans-Golgi network []. In addition, SNX6 was found to be translocated from the cytoplasm to nucleus by Pim-1, an oncogene product of serine/threonine kinase. This translocation is not affected by Pim-1-dependent phosphorylation, but the functional significance is unknown [].
This entry represents the C-terminal catalytic lipid kinase domain related to PtdInsP kinases (PIPKc domain) found in PIKfyve and related proteins.1-phosphatidylinositol-3-phosphate 5-kinase (), also called FYVE finger-containing phosphoinositide kinase (PIKfyve), forms a complex with its regulators, the scaffolding protein Vac14 and the lipid phosphatase Fig4. The complex synthesises phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P2]through the phosphorylation of phosphatidylinositol 3-phosphate (PtdIns3P or PI3P) on the fifth hydroxyl of the myo-inositol ring. Then phosphatidylinositol-5-phosphate (PtdIns5P) is generated directly from PtdIns(3,5)P2. PtdIns(3,5)P2 and PtdIns5P regulate endosomal trafficking and responses to extracellular stimuli []. It is vital in early embryonic development and plays a role in different pathways, such as receptor tyrosine kinase (RTK) or EGFR degradation, regulation of the glutamate transporters EAAT2, EAAT3 and EAAT4 and the cystic fibrosis transmembrane conductance regulator (CFTR). It is also essential for systemic glucose homeostasis and insulin-regulated glucose uptake/GLUT4 translocation in skeletal muscle [, , ].The yeast orthologue of human PIKfyve, Fab1, is required for endocytic-vacuolar pathway and nuclear migration [, ]. The plant orthologues such as FAB1A-D from Arabidopsis are important for the maintenance of endomembrane homeostasis and for development of viable pollen [, ].PIKfyve and its orthologues share a similar architecture consisting of a N-terminal FYVE domain, a middle region related to the CCT/TCP-1/Cpn60 chaperonins that are involved in productive folding of actin and tubulin, a second middle domain that contains a number of conserved cysteine residues (CCR) unique to these proteins, and a C-terminal catalytic lipid kinase domain related to PtdInsP kinases (or the PIPKc domain).
Fibulins area family of ECM glycoproteins characterized by a fibulin-type C-terminal domain preceded by tandem calcium-binding epidermal growth factor (EGF)-like modules. They are involved in protein-protein interaction with the components of basement membrane and extracellular matrix proteins. There are five fibulins, which can be classified into two subgroups. Fibulin-1 and -2 constitute one subgroup. These fibulins are larger than the others due to the presence of a higher number of EGF modules and an extra domain with three anaphylatoxin modules []. Members of the second subgroup, fibulin-3, -4, and -5, are similarly small in size and highly homologous to one another in modular structure. They consist of a modified cbEGF domain at the N terminus followed by five tandem cbEGF modules and the fibulin-type C-terminal region.EGF-containing fibulin-like extracellular matrix protein 1 (EFEMP1, also known as Fibulin-3) binds EGFR, the EGF receptor, inducing EGFR autophosphorylation and the activation of downstream signalling pathways []. It promotes glioma growth and resistance through a novel paracrine regulation of Notch signalling []and may function as a negative regulator of chondrocyte differentiation [].
The SH2-containing Shc adapter proteins are targets of activated tyrosine kinases and are implicated in the transmission of activation signals to the Ras/mitogen-activated protein kinase (MAPK) pathway []. Three Shc genes were originally identified in mammals that encode proteins characterised by an amino-terminal phosphotyrosine binding (PTB) domain and a carboxy-terminal Src homology 2 domain. Shc1 (ShcA) is ubiquitously expressed, whereas expression of Shc2 (ShcB) and Shc3 (ShcC) appears to be limited to neuronal cells [].A fourth Shc family protein, ShcD/Shc4, is expressed in adult brain and skeletal muscle. ShcD can associate via its PTB domain with the phosphorylated muscle-specific kinase (MuSK) receptor tyrosine kinase and undergo tyrosine phosphorylation downstream of activated MuSK. Therefore, ShcD may mediate a specific aspect of signalling downstream of the MuSK receptor []. ShcD also interacts with EGFR receptor (epidermal growth factor receptor) and facilitates its ligand-independent phosphorylation []. ShcD has been shown to be a modulator in the transition of embryonic stem cell (ESC) to epiblast stem cells (EpiSCs), the initial step for ESCs to commit to differentiation [].
This entry includes coiled-coil and C2 domain-containing protein 1A/B (CC2D1A/B, also known as Freud-1/2). CC2D1A is involved in many pathways, including nuclear factor kappaB, PDK1/Akt, cAMP/PKA, Notch and bone morphogenetic protein []. It is a calcium-regulated repressor of serotonine receptor 5-HT1A and dopamine-D2 receptor expression [, ]. CC2D1B binds to the 5-HT1A DRE and represses the human 5-HT1A receptor gene to regulate its expression in non-serotonergic cells and neurons [].CC2D1A and CC2D1B have also been shown to interact with the CHMP4 family of proteins, the major subunit of the ESCRT-III complex. They may regulate degradation and signaling of EGFR and TLR4 [].CC2D1A and CC2D1B share conserved domains, including several DM14 domains that are specific to this protein family, a C-terminal helix-loop-helix domain, and a C2 domain. The CC2D1A C2 domain is thought to be calcium insensitive and it lacks several acidic residues that mediate calcium binding of the PKC C2 domain. In addition, it contains a poly-basic insert that is not present in calcium-dependent C2 domains and may function as a nuclear localization signal []. The CC2D1B C2 domain appears to be essential for its DNA binding and repressor function; it may mediate protein-protein interactions []. Mutations in the CC2D1A gene has been linked to nonsyndromic mental retardation [, ].
RING finger protein 11 (RNF11) is an E3 ubiquitin-protein ligase that acts both as an adaptor and a modulator of itch-mediated control of ubiquitination events underlying membrane traffic. It is the downstream of an enzymatic cascade for the ubiquitination of specific substrates. It is also a molecular adaptor of homologous to E6-associated protein C terminus (HECT)-type ligases []. RNF11 has been implicated in the regulation of several signaling pathways. It enhances the transforming growth factor receptor (TGFR) signaling by both abrogating Smurf2-mediated receptor ubiquitination and by promoting the Smurf2-mediated degradation of AMSH (associated molecule with the SH3 domain of STAM), a de-ubiquitinating enzyme that enhances transforming growth factor-beta (TGF-beta) signalling and epidermal growth factor receptor (EGFR) endosomal recycling [, ]. It also acts directly on Smad4 to enhance Smad4 function, and plays a role in prolonged TGF-beta signalling []. Moreover, RNF11 functions as a critical component of the A20 ubiquitin-editing protein complex that negatively regulates tumor necrosis factor (TNF)-mediated nuclear factor (NF)-kappaB activation []. It also interacts with Smad anchor for receptor activation (SARA) and the endosomal sorting complex required for transport (ESCRT)-0 complex, thus participating in the regulation of lysosomal degradation of EGFR []. Furthermore, RNF11 acts as a novel GGA cargo actively participating in regulating the ubiquitination of the GGA protein family []. In addition, RNF11 functions together with TAX1BP1 to target TANK-binding kinase 1 (TBK1)/IkappaB kinase IKKi, and further restricts antiviral signaling and type I interferon (IFN)-beta production []. RNF11 contains an N-terminal PPPY motif that binds WW domain-containing proteins such as AIP4/itch, Nedd4 and Smurf1/2 (SMAD-specific E3 ubiquitin-protein ligase 1/2), and a C-terminal C3H2C3-type RING-H2 finger that functions as a scaffold for the coordinated transfer of ubiquitin to substrate proteins together with the E2 enzymes UbcH527 and Ubc13.
