CFBP Is a Novel Tyrosine-phosphorylated Protein That Might Function as a Regulator of CIN85/CD2AP

Behavioral analysis in mice deficient for GAREM2 (Grb2-associated regulator of Erk/MAPK subtype2) that is a subtype of highly expressing in the brain

Grb2-associated regulator of Erk/MAPK (GAREM), is an adaptor protein related to the several cell growth factor receptor-signaling. The GAREM family has two subtypes, GAREM1 and GAREM2, both encoded in the human and mouse genome. Recent genome-wide research identified GAREM2 as a candidate of neurodegenerative diseases. Here, we use knockout (KO) mice to show the role of GAREM2, that is highly expressed in the brain. According to the comprehensive behavioral battery, they exhibited less anxiety both in elevated plus maze and open field tests, mildly increased social approaching behavior in the reciprocal social interaction test, and longer latency to immobility in the tail suspension test as compared to wild-type (WT). Additionally, the extension of neurites in the primary cultured neurons was suppressed in ones derived from GAREM2 KO mice. Furthermore, we also identified Intersectin, as a binding partner of GAREM2 in this study. Intersectin is also a multi-domain adaptor protein that regulates endocytosis and cell signaling, which can potentially alter the subcellular localization of GAREM2. The important molecules, such as the neurotrophin receptor and Erk family, that are involved in the signaling pathway of the neural cell growth in the mouse brain, have been reported to participate in emotional behavior. As GAREM plays a role in the cellular growth factor receptor signaling pathway, GAREM2 may have a common role related to the transduction of Erk signaling in the higher brain functions.

MicroRNA-128 contributes to the progression of gastric carcinoma through GAREM-mediated MAPK signaling activation

Gastric carcinoma (GC) represents the most common malignant cancer and the second leading cause of cancer death worldwide. However, the molecular mechanisms and biological progression of GC remain unknown. In this study, we found that miR-128 is a critical tumor suppressor that is downregulated in GC patients and GC cells and that GAREM is a direct downstream target of miR-128. Overexpression of miR-128 in HGC-27 and MKN-45 cells resulted in suppressed cell growth and promoted cell apoptosis through a GAREM-dependent mechanism. Moreover, the precise mechanisms underlying the antitumor effect of miR-128 in GC are at least partially due to suppressing activation of the MAPK signaling pathway, induced by suppressing GAREM expression. This study is the first to demonstrate that the miR-128-GAREM-MAPK signaling pathway forms a critical feedback loop and mediates GC development, and these findings might demonstrate a potential therapeutic strategy for GC.

Versatile function of the circadian protein CIPC as a regulator of Erk activation

The CLOCK-interacting protein, Circadian (CIPC), has been identified as an additional negative-feedback regulator of the circadian clock. However, recent study on CIPC knockout mice has shown that CIPC is not critically required for basic circadian clock function, suggesting other unknown biological roles for CIPC. In this study, we focused on the cell cycle dependent nuclear-cytoplasmic shuttling function of CIPC and on identifying its binding proteins. Lys186 and 187 were identified as the essential amino acid residues within the nuclear localization signal (NLS) of CIPC. We identified CIPC-binding proteins such as the multifunctional enzyme CAD protein (carbamoyl-phosphate synthetase 2, aspartate transcarbamoylase, and dihydroorotase), which is a key enzyme for de novo pyrimidine synthesis. Compared to control cells, HEK293 cells overexpressing wild-type CIPC showed suppressed cell proliferation and retardation of cell cycle. We also found that PMA-induced Erk activation was inhibited with expression of wild-type CIPC. In contrast, the NLS mutant of CIPC, which reduced the ability of CIPC to translocate into the nucleus, did not exhibit these biological effects. Since CAD and Erk have significant roles in cell proliferation and cell cycle, CIPC may work as a cell cycle regulator by interacting with these binding proteins.

Functional relationship between CABIT, SAM and 14-3-3 binding domains of GAREM1 that play a role in its subcellular localization

GAREM1 (Grb2-associated regulator of Erk/MAPK1) is an adaptor protein that is involved in the epidermal growth factor (EGF) pathway. The nuclear localization of GAREM1 depends on the nuclear localization sequence (NLS), which is located at the N-terminal CABIT (cysteine-containing, all in Themis) domain. Here, we identified 14-3-3ε as a GAREM-binding protein, and its binding site is closely located to the NLS. This 14-3-3 binding site was of the atypical type and independent of GAREM phosphorylation. Moreover, the binding of 14-3-3 had an effect on the nuclear localization of GAREM1. Unexpectedly, we observed that the CABIT domain had intramolecular association with the C-terminal SAM (sterile alpha motif) domain. This association might be inhibited by binding of 14-3-3 at the CABIT domain. Our results demonstrate that the mechanism underlying the nuclear localization of GAREM1 depends on its NLS in the CABIT domain, which is controlled by the binding of 14-3-3 and the C-terminal SAM domain. We suggest that the interplay between 14-3-3, SAM domain and CABIT domain might be responsible for the distribution of GAREM1 in mammalian cells.

The molecular basis for selective assembly of the UBAP1-containing endosome-specific ESCRT-I complex

ESCRT-I is essential for the multivesicular body (MVB) sorting of ubiquitylated cargo such as epidermal growth factor receptor, as well as for several cellular functions, such as cell division and retroviral budding. ESCRT-I has four subunits; TSG101, VPS28, VPS37 and MVB12. There are several members of VPS37 and MVB12 families in mammalian cells, and their differential incorporation into ESCRT-I could provide function-specific variants of the complex. However, it remains unclear whether these different forms of VPS37 and MVB12 combine randomly or generate selective pairings within ESCRT-I, and what the mechanistic basis for such pairing would be. Here, we show that the incorporation of two MVB12 members, UBAP1 and MVB12A, into ESCRT-I is highly selective with respect to their VPS37 partners. We map the region mediating selective assembly of UBAP1-VPS37A to the core ESCRT-I-binding domain of VPS37A. In contrast, selective integration of UBAP1 requires both the minimal ESCRT-I-binding region and a neighbouring predicted helix. The biochemical specificity in ESCRT-I assembly is matched by functional specialisation as siRNA-mediated depletion of UBAP1, but not MVB12A and MVB12B, disrupts ubiquitin-dependent sorting at the MVB.

A brain-specific Grb2-associated regulator of extracellular signal-regulated kinase (Erk)/mitogen-activated protein kinase (MAPK) (GAREM) subtype, GAREM2, contributes to neurite outgrowth of neuroblastoma cells by regulating Erk signaling

Grb2-associated regulator of Erk/MAPK1 (GAREM) is an adaptor molecule in the EGF-mediated signaling pathway. GAREM is expressed ubiquitously in human organs and cultured cells. Two GAREM homologues are encoded by the human genome. Therefore, previously identified GAREM is named GAREM1. Here we characterized a new subtype of GAREM, GAREM2, that is specifically expressed in the mouse, rat, and human brain. Three GAREM2 tyrosines (Tyr-102, Tyr-429, and Tyr-551) are phosphorylated upon EGF stimulation and are necessary for binding to Grb2. Furthermore, GAREM2 and Shp2 regulate Erk activity in EGF-stimulated cells. These characteristics are similar to those of GAREM1. GAREM2 is expressed in some neuroblastoma cell lines and is also tyrosine-phosphorylated and bound to Grb2 after treatment with EGF. Eventually, GAREM2 regulates Erk activation in the presence of EGF or insulin like growth factor 1. GAREM2 also regulates insulin-like growth factor 1-induced neuronal differentiation of the SH-SY5Y neuroblastoma cell line. Although the structure and function of both GAREM subtypes are similar, GAREM1 is recruited into the nucleus and GAREM2 is not. Nuclear localization of GAREM1 might be controlled by a GAREM1-specific nuclear localization sequence and 14-3-3ε binding. The N-terminal 20 amino acids of GAREM1 make up its nuclear localization sequence that is also a 14-3-3ε binding site. The GAREM family is a new class of adaptor molecules with subtype-specific biological functions.

Inhibitory effect of SPE-39 due to tyrosine phosphorylation and ubiquitination on the function of Vps33B in the EGF-stimulated cells

Although SPE-39 is a binding protein to Vps33B that is one of the subunit in the mammalian HOPS complex, the elements of SPE-39 function remain unknown. Here, we show that tyrosine phosphorylation of SPE-39 following EGF stimulation plays a role in the stability of SPE-39 itself. Ubiquitination of the C-terminal region of SPE-39 was also elevated in response to EGF stimulation, and this process was regulated by the phosphorylation of Tyr-11 in SPE-39. However, association of Vps33B with SPE-39 inhibited the elevation of ubiquitination of SPE-39 following EGF stimulation, which might be responsible for the stabilization of SPE-39. Furthermore, an opposing functional relationship between SPE-39 and Vps33B on the downregulation of the EGF receptor was observed in EGF-stimulated COS-7 cells.

CD2AP regulates SUMOylation of CIN85 in podocytes

Podocytes are highly differentiated and polarized epithelial cells located on the visceral side of the glomerulus. They form an indispensable component of the glomerular filter, the slit diaphragm, formed by several transmembrane proteins and adaptor molecules. Disruption of the slit diaphragm can lead to massive proteinuria and nephrotic syndrome in mice and humans. CD2AP is an adaptor protein that is important for the maintenance of the slit diaphragm. Together with its paralogue, CIN85, CD2AP belongs to a family of adaptor proteins that are primarily described as being involved in endocytosis and downregulation of receptor tyrosine kinase activity. We have shown that full-length CIN85 is upregulated in podocytes in the absence of CD2AP, whereas in wild-type cells, full-length CIN85 is not detectable. In this study, we show that full-length CIN85 is postranslationally modified by SUMOylation in wild-type podocytes. We can demonstrate that CIN85 is SUMOylated by SUMO-1, -2, and -3 and that SUMOylation is enhanced in the presence of CD2AP. Conversion of lysine 598 to arginine completely abolishes SUMOylation and leads to increased binding of CIN85 to nephrin. Our results indicate a novel role for CD2AP in regulating posttranslational modification of CIN85.

Regulation of CD2-associated protein influences podocyte endoplasmic reticulum stress-mediated apoptosis induced by albumin overload

Proteinuria is an exacerbating factor of chronic kidney diseases, leading to glomerulosclerosis. However, the molecular mechanisms mediating protein overload-induced podocyte injury are poorly understood. Recent studies have shown that apoptosis mediated by endoplasmic reticulum (ER) stress participated in the progression of a variety of kidney diseases. In the present study, we investigated the role of CD2-associated protein (CD2AP) in protein overload-induced ER stress and subsequent podocyte apoptosis. Conditionally immortalized mouse podocytes were cultured in vitro and treated with different concentrations of bovine serum albumin (BSA). In addition, CD2AP eukaryotic expression vector or siRNA was transfected into podocytes before exposed to BSA. Albumin endocytosis and podocyte apoptosis were visualized by confocal microscopy. The subcellular organelles were observed by transmission electron microscopy. The expressions of GRP78, caspase-12 and CD2AP were detected by RT-PCR or Western blot analysis. It was found that albumin was endocytosed by podocytes in a time-dependent manner. Accumulation of albumin in podocytes induced ER stress and apoptosis in a concentration-dependent manner as indicated by upregulation of GRP78 and caspase-12. Meanwhile, the subcellular organelles were disrupted and the expression of CD2AP was downregulated by high concentration of albumin. Transfection of CD2AP eukaryotic expression vector into podocytes increased CD2AP expression, depressed GRP78 and caspase-12 expressions, and inhibited podocyte apoptosis. In contrast, transfection of CD2AP siRNA deteriorated the above changes induced by BSA. It is concluded protein overload induces podocyte apoptosis via ER stress and CD2AP may play a crucial role in albumin overload-induced ER stress and apoptosis in podocytes.

Role of a tyrosine phosphorylation of SMG-9 in binding of SMG-9 to IQGAP and the NMD complex

SMG-9 is a component of the NMD complex, a heterotetramer that also includes SMG-1 and SMG-8 in the complex. SMG-9 was also originally identified as a tyrosine-phosphorylated protein but the role of the phosphorylation is not yet known. In this study, we determined that IQGAP protein, an actin cytoskeleton modifier acts as a binding partner with SMG-9 and this binding is regulated by phosphorylation of SMG-9 at Tyr-41. SMG-9 is co-localized with IQGAP1 as a part of the process of actin enrichment in non-stimulated cells, but not in the EGF-stimulated cells. Furthermore, an increase in the ability of SMG-9 to bind to SMG-8 occurs in response to EGF stimulation. These results suggest that tyrosine phosphorylation of SMG-9 may play a role in the formation of the NMD complex in the cells stimulated by the growth factor.

UMA and MABP domains throw light on receptor endocytosis and selection of endosomal cargoes

Interactions of the ESCRT complexes are critical for endosomal trafficking. We identify two domains with potential significance for this process. The MABP domain present in metazoan ESCRT-I/MVB12 subunits, Crag, a regulator of protein sorting, and bacterial pore-forming proteins might mediate novel membrane interactions in trafficking. The UBAP1-MVB12-associated UMA domain found in MVB12 and UBAP1 defines a novel adaptor that might recruit diverse targets to ESCRT-I.

Contact:aravind@ncbi.nlm.nih.gov

Supplementary information:Supplementary data are available at ftp://ftp.ncbi.nih.gov/pub/aravind/UMA/MVB12.html.

[New functional proteins identified by proteomic analysis in the epidermal growth factor receptor-mediated signaling pathway and application for practical use]

To clarify the whole picture of epidermal growth factor (EGF) signaling pathway, we identified proteins from the EGF-stimulated A431 cells by anti-phospho-tyrosine antibody column chromatography. Over 150 proteins were detected including previously unidentified proteins as well as well-studied proteins. Among these proteins, we picked up four proteins that had not been known in EGF signaling pathway and analyzed their functions. We report the functions of these proteins in this article. 1) CFBP interacts with CD2AP family proteins and functions as a key component in downregulation of EGF receptor protein level following EGF stimulation. 2) Ymer is found to be phosphorylated and ubiquitinated upon EGF stimulation, and functions as a regulator for the downregulation and endocytosis of EGF receptor. 3) CLPABP binds to mitochondria-specific phospholipids, cardiolipin, through its PH domain, and its complex includes various proteins related to mRNA metabolism. 4) GAREM is associated with Grb2 and Shp2. Each association affects the ERK activity. Finally, we discuss the possibilities that these proteins can be used as a novel biomarker protein for cancer and other diseases.

Emerging roles of Ruk/CIN85 in vesicle-mediated transport, adhesion, migration and malignancy

Ruk/CIN85 is an adaptor protein. Similar to many other proteins of this type, Ruk/CIN85 is known to take part in multiple cellular processes including signal transduction, vesicle-mediated transport, cytoskeleton remodelling, programmed cell death and viral infection. Recent studies have also revealed the potential importance of Ruk/CIN85 in cancer cell invasiveness. In this review we summarize the various roles of this protein as well as the potential contribution of Ruk/CIN85 to malignancy and the invasiveness of cancer cells. In the last section of the paper we also speculate on the utility of Ruk/CIN85 as a target for novel anti-cancer therapies.

GAREM, a novel adaptor protein for growth factor receptor-bound protein 2, contributes to cellular transformation through the activation of extracellular signal-regulated kinase signaling

Adaptor proteins for the various growth factor receptors play a crucial role in signal transduction through tyrosine phosphorylation. Several candidates for adaptor proteins with potential effects on the epidermal growth factor (EGF) receptor-mediated signaling pathway have been identified by recent phosphoproteomic studies. Here, we focus on a novel protein, GAREM (Grb2-associated and regulator of Erk/MAPK) as a downstream molecule of the EGF receptor. GAREM is phosphorylated at tyrosine 105 and 453 after EGF stimulation. Grb2 was identified as its binding partner, and the proline-rich motifs of GAREM are recognized by the N- and C-terminal SH3 domains of Grb2. In addition, the tyrosine phosphorylations of GAREM are necessary for its binding to Grb2. Because the amino acid sequence surrounding tyrosine 453 is similar to the immunoreceptor tyrosine-based inhibitory motif, Shp2, a positive regulator of Erk, binds to GAREM in this phosphorylation-dependent manner. Consequently, Erk activation in response to EGF stimulation is regulated by the expression of GAREM in COS-7 and HeLa cells, which occurs independent of the presence of other binding proteins, such as Gab1 and SOS, to the activated EGF receptor. Furthermore, the expression of GAREM has an effect on the transformation activity of cultured cells. Together, these findings suggest that GAREM plays a key role in the ligand-mediated signaling pathway of the EGF receptor and the tumorigenesis of cells.

Intersectin 1 forms a complex with adaptor protein Ruk/CIN85 in vivo independently of epidermal growth factor stimulation

Intersectin 1 (ITSN1) is an adaptor protein involved in clathrin-mediated endocytosis, apoptosis, signal transduction and cytoskeleton organization. Here, we show that ITSN1 forms a complex with adaptor protein Ruk/CIN85, implicated in downregulation of receptor tyrosine kinases. The interaction is mediated by the SH3A domain of ITSN1 and the third or fourth proline-rich blocks of Ruk/CIN85, and does not depend on epidermal growth factor stimulation, suggesting a constitutive association of ITSN1 with Ruk/CIN85. Moreover, both proteins colocalize in MCF-7 cells with their common binding partner, the ubiquitin ligase c-Cbl. The possible biological role of the interaction between ITSN1 and Ruk/CIN85 is discussed.

CD2-associated protein is widely expressed and differentially regulated during embryonic development

CD2-associated protein (CD2AP) is an adapter protein that is involved in various signaling and vesicular trafficking processes and also functions as a linker between plasma membrane proteins and the actin cytoskeleton. The protein is known to have important functions in T cells and glomerular podocytes, but it is also expressed by many other adult-type tissues and cells. Here we analyzed the expression of the protein during early embryonic development and organogenesis of the mouse. The results showed differential tissue-specific regulation of CD2AP in developing and maturing organs. In oocytes and pre-implantation embryos, CD2AP was located diffusely in the cytoplasm, whereas in late blastocysts it was concentrated to the intercellular contacts. During organogenesis, CD2AP was distinctly upregulated upon, e.g., the pretubular aggregation of metanephric mesenchyme cells and the appearance of the osteoblastic rim around cartilages during endochondral ossification. High CD2AP expression was also observed during epithelial-like conversion of some highly specialized secretory cell types such as the odontoblasts, the cells of the choroid plexus and the decidualized cells of the endometrial stroma. In other instances, such as the development of the proximal tubuli of the kidney and the flat alveolar epithelium of the lung, the protein was downregulated upon differentiation and maturation of the cells. Finally, certain cells, e.g., glomerular podocytes, those forming the collecting ducts of the kidney, and the urothelium of the kidney pelvis, expressed CD2AP throughout their differentiation and maturation. Multiple molecules and complex pathways regulate embryogenesis, and scaffolding proteins apparently have pivotal roles in targeting and finetuning, e.g., growth factor- or hormone-induced processes. The cell-type specific spatio-temporal regulation of CD2AP during development suggests that this adapter protein is a key regulatory partner in many signaling pathways and cellular processes governing differentiation and morphogenesis.

Rab proteins in endocytosis and Glut4 trafficking

The intracellular trafficking of numerous proteins requires a tight control to fulfil their physiological functions. It is the case of the adipocyte and muscle glucose transporter Glut4 that is retained intracellularly until insulin induces its recruitment to the plasma membrane. Rabs are evolutionarily conserved small GTPases that control intracellular traffic events from yeast to mammalian cells. In the past few decades, considerable progresses have been made in identifying the route of Glut4, the Rabs involved in controlling it, and more recently the connection between insulin signalling and Glut4 trafficking through Rab activity control.

MVB-12, a fourth subunit of metazoan ESCRT-I, functions in receptor downregulation

After ligand binding and endocytosis, cell surface receptors can continue to signal from endosomal compartments until sequestered from the cytoplasm. An important mechanism for receptor downregulation in vivo is via the inward budding of receptors into intralumenal vesicles to form specialized endosomes called multivesicular bodies (MVBs) that subsequently fuse with lysosomes, degrading their cargo. This process requires four heterooligomeric protein complexes collectively termed the ESCRT machinery. In yeast, ESCRT-I is a heterotetrameric complex comprised of three conserved subunits and a fourth subunit for which identifiable metazoan homologs were lacking. Using C. elegans, we identify MVB-12, a fourth metazoan ESCRT-I subunit. Depletion of MVB-12 slows the kinetics of receptor downregulation in vivo, but to a lesser extent than inhibition of other ESCRT-I subunits. Consistent with these findings, targeting of MVB-12 to membranes requires the other ESCRT-I subunits, but MVB-12 is not required to target the remaining ESCRT-I components. Both endogenous and recombinant ESCRT-I are stable complexes with a 1:1:1:1 subunit stoichiometry. MVB-12 has two human homologs that co-localize and co-immunoprecipitate with the ESCRT-I component TSG101. Thus, MVB-12 is a conserved core component of metazoan ESCRT-I that regulates its activity during MVB biogenesis.

Focal segmental glomerulosclerosis in a patient homozygous for a CD2AP mutation

Focal segmental glomerulosclerosis (FSGS) is a histologic diagnosis in several kidney diseases characterized by proteinuria and a severe decrease in kidney function. Mutations in several genes were found in patients with primary FSGS, one of which is a CD2-associated protein CD2AP (originally referred to as CMS). This gene encodes an adaptor protein that plays a role in endocytosis, cell motility, and cell survival. Mice deficient in Cd2ap (the mouse homolog) die due to kidney failure, while heterozygous mice develop lesions similar to those of FSGS patients. In the kidney, CD2AP regulates the actin cytoskeleton. The only previously described patient with CD2AP mutation had a severely truncated protein. In this study, we describe a patient with a novel mutation resulting in a premature stop codon yielding a protein truncated by only 4%. This shortened CD2AP protein displays a significantly decreased F-actin binding efficiency in vitro with no expression of the mutated allele in the patient's lymphocytes. Heterozygous expression of the CD2AP mutation in both parents did not lead to any kidney pathology, as both have normal glomerular filtration rates and no proteinuria.