A missense polymorphism (H204R) of a Rho GTPase-activating protein, the chimerin 2 gene, is associated with schizophrenia in men.
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Arginine↗
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Biochemical comparisons of the Saccharomyces cerevisiae Bem2 and Bem3 proteins. Delineation of a limit Cdc42 GTPase-activating protein domain.
The Bem2 and Bem3 proteins, which appear to play roles in the regulation of bud site formation in Saccharomyces cerevisiae, show striking homology to a number of proteins that compose a family of GTPase-activating proteins (GAPs) for the rho-subgroup of ras-related GTP-binding proteins. These members include human platelet GAP for Cdc42Hs (the human homolog of a S. cerevisiae GTP-binding protein that regulates bud site assembly), the break point cluster region protein, the brain protein chimerin, the 85-kDa regulatory subunit (p85) of the phosphatidylinositol 3-kinase, and the ras-GAP-binding protein (p190). A fusion protein composed of the glutathione S-transferase protein and the rho-GAP homology region of Bem3 (designated GST-Bem3) stimulates the GTPase activity of the wild-type Cdc42Hs protein (Cdc42HsGly-12), but has no stimulatory effect on a GTPase-defective mutant (Cdc42HsVal-12), whereas a GST-Bem2 fusion protein does not stimulate the GTPase activity of either form of Cdc42Hs. We have compared the ability of GST-Bem3 to serve as a GAP for Cdc42Hs relative to other members of the rho-GAP subfamily and found the following order of potency: human platelet Cdc42Hs GAP > p190 > Bem3 > break point cluster region protein, whereas p85, like Bem2, shows no GAP activity or any ability to bind to the GTP-bound form of Cdc42Hs. We have taken advantage of the functional specificity exhibited by Bem3 (versus Bem2) in using Bem2/Bem3 chimeras, as well as different deletion mutant versions of the Bem3 protein, to delineate the limits of a functional Cdc42 GAP domain. The results of this study indicate that the carboxyl-terminal approximately 224 amino acids (which contain three regions of homology to the other members of the rho-GAP family) represent a "limit GAP." The first two appear to be important for binding to Cdc42Hs and for partial GAP activity.
Amino Acid Sequence↗
Chimaerins, novel non-protein kinase C phorbol ester receptors, associate with Tmp21-I (p23): evidence for a novel anchoring mechanism involving the chimaerin C1 domain.
The regulation and function of chimaerins, a family of "non-protein kinase C" (PKC) phorbol ester/diacylglycerol receptors with Rac-GAP activity, is largely unknown. In a search for chimaerin-interacting proteins, we isolated Tmp21-I (p23), a protein localized at the perinuclear Golgi area. Remarkably, phorbol esters translocate beta2-chimaerin to the perinuclear region and promote its association with Tmp21-I in a PKC-independent manner. A deletional analysis revealed that the C1 domain in chimaerins is required for the interaction with Tmp21-I, thereby implying a novel function for this domain in protein-protein associations in addition to its role in lipid and phorbol ester binding. Our results support the emerging concept that multiple pathways transduce signaling by phorbol esters and revealed that, like PKC isozymes, chimaerins are subject to a positional regulation. In this setting, Tmp21-I serves as an anchoring protein that determines the intracellular localization of these novel phorbol ester receptors.
Amino Acid Sequence↗
Target-assisted iterative screening reveals novel interactors for PSD95, Nedd4, Src, Abl and Crk proteins.
A new in vitro screening method has been developed and applied to a commercial phage-displayed cDNA library to search for novel protein-protein interactions. PDZ, WW and SH3 domains from PSD95, Nedd4, Src, Abl and Crk proteins were used as targets. 12 novel putative and 2 previously reported interactions were identified in test screens. The novel screening format, dubbed TAIS (target-assisted iterative screening), is discussed as an alternative platform to existing technologies for a pair-wise characterization of protein-protein interactions.
Amino Acid Sequence↗
EBV can protect latently infected B cell lymphomas from death receptor-induced apoptosis.
The relationship between EBV infection and sensitivity to death receptor (DR)-induced apoptosis is poorly understood. Using EBV- and EBV+ BJAB cells, we provide the first evidence that EBV can protect latently infected B cell lymphomas from apoptosis triggered through Fas or TRAIL receptors. Caspase 8 activation was impaired and cellular FLIP recruitment was enriched in death-inducing signaling complexes formed in EBV-infected BJAB cells relative to parent BJAB cells. Furthermore, latent membrane protein 1 expression alone could reduce caspase activation and confer partial resistance to DR apoptosis in BJAB cells. This protective effect was dependent on C-terminal activating region 2-driven NF-kappaB activation, which in turn up-regulated cellular FLIP expression in latent membrane protein 1+ BJAB cells. Thus, the ability of latent EBV to block DR apoptosis may help to ensure the survival of host cells during B cell differentiation, and contribute to the development of B cell lymphomas, especially in immunocompromised individuals.
Apoptosis↗
Alpha 2-chimerin, an SH2-containing GTPase-activating protein for the ras-related protein p21rac derived by alternate splicing of the human n-chimerin gene, is selectively expressed in brain regions and testes.
n-Chimerin (alpha 1-chimerin) is a brain GTPase-activating protein (GAP) for the ras-related p21rac. We now report the occurrence of another form of chimerin, termed alpha 2-chimerin. This is the product of an alternately spliced transcript of the human n-chimerin gene encoding an N-terminal SH2 (src homology 2) domain in addition to the phorbol ester receptor and GAP domains. alpha 1- and alpha 2-chimerin mRNAs were expressed differently. In the rat brain, only alpha 1-chimerin mRNA was expressed in cerebellar Purkinje cells, although both alpha 1- and alpha 2-chimerin mRNAs occurred in neurons in the cerebral cortex, hippocampus, and thalamus. Only alpha 2-chimerin RNA was expressed in rat testes, in early pachytene spermatocytes. A 45-kDa SH2-containing chimerin corresponding to the alpha 2 form was purified from rat brain. As with Escherichia coli 45-kDa recombinant alpha 2-chimerin, purified brain alpha 2-chimerin exhibited racGAP activity which was stimulated by phosphatidylserine. The recombinant SH2 domain bound several 32P-labelled phosphoproteins of PC12 cells, whose phosphorylation increased in response to trophic factors, including nerve growth factor. To examine the relationships of alpha 1- and alpha 2-chimerin transcripts, human genomic DNA clones were characterized. In alpha 2-chimerin mRNA, a 3' splice acceptor site within exon 1 of alpha 1-chimerin mRNA was used, replacing its 5' untranslated region and N-terminal coding sequence. The single human n-chimerin gene was mapped to chromosome 2q31-q32.1, colocalizing with the CRE-BP1 transcription factor gene (2q32). It contained several splice junctions conserved with the sequence-related protein kinase C and bcr genes. alpha 2-Chimerin is only the second SH2-containing GAP and the first example of an SH2 domain generated by alternate splicing.
Alternative Splicing↗
Solution structure of a cysteine rich domain of rat protein kinase C.
Intracellular protein phosphorylation by protein kinase C (PKC) plays a major role in the translation of extracellular signals into cellular events. Speculations on the structural basis for PKC activation are based on sequence homology between their cysteine-rich domains (CRD) and the DNA-binding 'zinc-fingers'. We produced a fragment comprising the second CRD (CRD2) of rat PKC-alpha and determined its three-dimensional structure in solution by NMR spectroscopy. This revealed that CRD2 adopts a globular fold allowing two non-consecutive sets of zinc-binding residues to form two separate metal-binding sites. The fold is different to those previously proposed and allows insight into the molecular topology of a family of homologous proteins.
Amino Acid Sequence↗
GTPase-activating protein activity of n(alpha 1)-Chimaerin and effect of lipids.
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ATP-Binding Cassette Transporters↗
The GTPase-activating protein n-chimaerin cooperates with Rac1 and Cdc42Hs to induce the formation of lamellipodia and filopodia.
n-Chimaerin is a GTPase-activating protein (GAP) mainly for Rac1 and less so for Cdc42Hs in vitro. The GAP activity of n-chimaerin is regulated by phospholipids and phorbol esters. Microinjection of Rac1 and Cdc42Hs into mammalian cells induces formation of the actin-based structures lamellipodia and filopodia, respectively, with the former being prevented by coinjection of the chimaerin GAP domain. Strikingly, microinjection of the full-length n-chimaerin into fibroblasts and neuroblastoma cells induces the simultaneous formation of lamellipodia and filopodia. These structures undergo cycles of dissolution and formation, resembling natural morphological events occurring at the leading edge of fibroblasts and neuronal growth cones. The effects of n-chimaerin on formation of lamellipodia and filopodia were inhibited by dominant negative Rac1(T17N) and Cdc42Hs(T17N), respectively. n-Chimaerin's effects were also inhibited by coinjection with Rho GDP dissociation inhibitor or by treatment with phorbol ester. A mutant n-chimaerin with no GAP activity and impaired p21 binding was ineffective in inducing morphological changes, while a mutant lacking GAP activity alone was effective. Microinjected n-chimaerin colocalized in situ with F-actin. Taken together, these results suggest that n-chimaerin acts synergistically with Rac1 and Cdc42Hs to induce actin-based morphological changes and that this action involves Rac1 and Cdc42Hs binding but not GAP activity. Thus, GAPs may have morphological functions in addition to downregulation of GTPases.
3T3 Cells↗
Human brain n-chimaerin cDNA encodes a novel phorbol ester receptor.
A human brain-specific cDNA encoding n-chimaerin, a protein of predicted molecular mass 34 kDa, has sequence identity with two different proteins: protein kinase C (PKC) at the N-terminus and BCR protein [product of the breakpoint-cluster-region (BCR) gene, involved in Philadelphia chromosome translocation] at the C-terminus [Hall, Monfries, Smith, Lim, Kozma, Ahmed, Vannaisungham, Leung & Lim (1990) J. Mol. Biol. 211, 11-16]. The sequence identity of n-chimaerin with PKC includes the cysteine-rich motif CX2CX13CX2CX7CX7C, and amino acids upstream of the first cysteine residue, but not the kinase domain. This region of PKC has been implicated in the binding of diacylglycerol and phorbol esters in a phospholipid-dependent fashion. Part of this cysteine-rich motif (CX2CX13CX2C) has the potential of forming a 'Zn-finger' structure. Phorbol esters cause a variety of physiological changes and are among the most potent tumour promoters that have been described. PKC is the only known protein target for these compounds. We now report that n-chimaerin cDNA encodes a novel phospholipid-dependent phorbol ester receptor, with the cysteine-rich region being responsible for this activity. This finding has wide implications for previous studies equating phorbol ester binding with the presence of PKC in the brain.
Amino Acid Sequence↗
Tumor promoter binding of the protein kinase C C1 homology domain peptides of RasGRPs, chimaerins, and Unc13s.
Recent investigations discovered nonkinase-type phorbol ester receptors, RasGRPs, chimaerins, and Unc13s. Phorbol ester binding occurs at the cysteine-rich sequences of about 50 residues in the C1 domains of these receptors. Fifty-one-residue RasGRP C1 peptides except for RasGRP2 showed significant phorbol 12,13-dibutyrate (PDBu) binding, but the K(d) values of the RasGRP1 and RasGRP3 C1 peptides were about 10-fold larger than those for the corresponding whole enzymes. Addition of the C-terminal basic amino acid cluster decreased their K(d) values about 10-fold, suggesting that the positive charges of these C1 peptides play an important role in the PDBu binding in the presence of negatively-charged phosphatidylserine. The 51-mer chimaerin C1 peptides showed potent PDBu binding, while the Unc13 and Munc13-1 C1 peptides without sufficient positive charges hardly bound PDBu. By the rapid screening system using this C1 peptide library, 5-prenyl-indolactam-V was identified as a promising lead for the novel protein kinase C isozyme specific ligands.
Amino Acid Sequence↗
C1 domains exposed: from diacylglycerol binding to protein-protein interactions.
C1 domains, cysteine-rich modules originally identified in protein kinase C (PKC) isozymes, are present in multiple signaling families, including PKDs, chimaerins, RasGRPs, diacylglycerol kinases (DGKs) and others. Typical C1 domains bind the lipid second messenger diacylglycerol (DAG) and DAG-mimetics such as phorbol esters, and are critical for governing association to membranes. On the contrary, atypical C1 domains possess structural determinants that impede phorbol ester/DAG binding. C1 domains are generally expressed as twin modules (C1A and C1B) or single domains. Biochemical and cellular studies in PKC and PKD isozymes revealed that C1A and C1B domains are non-equivalent as lipid-binding motifs or translocation modules. It has been recently determined that individual C1 domains have unique patterns of ligand recognition, driven in some cases by subtle structural differences. Insights from recent 3-D studies on beta2-chimaerin and Munc13-1 revealed that their single C1 domains are sterically blocked by intramolecular interactions, suggesting that major conformational changes would be required for exposing the site of DAG interaction. Thus, it is clear that the protein context plays a major role in determining whether binding of DAG to the C1 domain would lead to enzyme activation or merely serves as an anchoring mechanism.
Amino Acid Sequence↗
[Characterization of new chemoattractant agents in leukocytes].
G protein-coupled receptors constitute one of the largest gene families in mammals. About a hundred orphan receptors still exist, for which the ligands and functions are unknown. We have recently identified the natural ligands of two orphan receptors expressed in dendritic cells and monocytes/macrophages. Chemerin, product of the gene Tig-2, was characterized as the ligand of the chemR23 receptor. The protein is synthesized as an inactive precursor, prochemerin, which requires the proteolytic removal of the last 6 or 7 amino acids, in order to generate a high affinity ligand of chemR23. Two neutrophil proteases, elastase and cathepsin G, are able to mediate this conversion. Besides, a peptide derived from the intracellular protein heme-binding protein (HBP) has been characterized as the first specific ligand of the FPRL2 receptor. The role of these two new systems in the control of physiological and pathological inflammatory reactions is presently being studied.
Amino Acid Sequence↗
Engineered protein cages for nanomaterial synthesis.
Self-assembled particles of genetically engineered human L subunit ferritin expressing a silver-binding peptide were used as nanocontainers for the synthesis of silver nanoparticles. The inner cavity of the self-assembled protein cage displays a dodecapeptide that is capable of reducing silver ions to metallic silver. This chimeric protein cage when incubated in the presence of silver nitrate exhibits the growth of a silver nanocrystal within its cavity. Our studies indicate that it is possible to design chimeric cages, using specific peptide templates, for the growth of other inorganic nanoparticles.
Amino Acid Sequence↗
Molecular heterogeneity and function of EWS-WT1 fusion transcripts in desmoplastic small round cell tumors.
Desmoplastic small round cell tumor (DSRCT) is a primitive sarcoma with a consistent cytogenetic abnormality, t(11;22)(p13;q12). This chromosomal translocation generates a chimeric transcript that is formed by fusion of the 5' region of the Ewing's sarcoma gene, EWS, with the 3' DNA-binding segment of WT1, the Wilms' tumor suppressor gene. We collected 14 DSRCT tumor samples and examined the hybrid transcripts. We identified: (a) combinatorial heterogeneity of EWS exons fused to WT1 including use of EWS exons 7, 8, and 9; (b) subpopulations of variant transcripts in 6 of 14 tumors characterized by aberrant splicing resulting in loss of EWS exon 6 or WT1 exon 9; (c) multiple cDNA products with large internal deletions; and (d) insertion of small stretches of heterologous DNA at the fusion site or exon splice region in transcripts from two tumors. Most of the splice variants were in-frame, and in vitro translated fusion proteins with intact DNA-binding motifs formed complexes with a WT1 response element in gel mobility assays. Each of the chimeric proteins retains the ability to bind to the GC and TC elements of the early transcription factor EGR-1 as well as WT1 consensus sequences. We present evidence that various EWS-WT1 proteins up-regulated EGR-1 promoter activity and that this up-regulation is specifically dependent upon the absence of the exon 9 KTS domain of WT1. The molecular diversity and functionality exhibited by these fusion transcripts may have significant biological implications for their transactivating and tumorigenic potential.
Abdominal Neoplasms↗
Move over protein kinase C, you've got company: alternative cellular effectors of diacylglycerol and phorbol esters.
Diacylglycerol is an essential second messenger in mammalian cells. The most prominent intracellular targets of diacylglycerol and of the functionally analogous phorbol esters belong to the protein kinase C (PKC) family. However, at least five alternative types of high-affinity diacylglycerol/phorbol-ester receptor are known: chimaerins, protein kinase D, RasGRPs, Munc13s and DAG kinase gamma. Recent evidence indicates that these have functional roles in diacylglycerol second messenger signalling in vivo and that several cellular processes depend on these targets rather than protein kinase C isozymes. These findings contradict the still prevalent view according to which all diacylglycerol/phorbol-ester effects are caused by the activation of protein kinase C isozymes. RasGRP1 (in Ras/Raf/MEK/ERK signalling) and Munc13-1 (in neurotransmitter secretion) are examples of non-PKC diacylglycerol/phorbol-ester receptors that mediate diacylglycerol and phorbol-ester effects originally thought to be caused by PKC isozymes. In the future, pharmacological studies on PKC must be complemented with alternative experimental approaches to allow the separation of PKC-mediated effects from those caused by alternative targets of the diacylglycerol second messenger pathway. The examples of RasGRP1 and Munc13-1 show that detailed genetic analyses of C(1)-domain-containing non-PKC diacylglycerol/phorbol-ester receptors in mammals are ideally suited to achieve this goal.
Animals↗
The exosome pathway in K562 cells is regulated by Rab11.
During maturation, reticulocytes lose some membrane proteins that are not required on the mature red cell surface. The proteins are released into the extracellular medium associated with vesicles that are formed by budding of the endosomal membrane into the lumen of the compartment; this process results in the formation of multivesicular bodies (MVBs). Fusion of MVBs with the plasma membrane results in secretion of the small internal vesicles, termed exosomes. K562 cells release exosomes with similar characteristics to reticulocyte exosomes, in particular the transferrin receptor (TfR) is found associated with the vesicles. Interestingly, this cell line has been shown to possess high amounts of Rab11 compared with other Rab proteins. To assess the regulation of transferrin receptor release via exosome secretion by Rab11 in this cell type, K562 cells were stably transfected with GFP-Rab11wt or the GTP- and GDP-locked mutants. The distribution of the proteins was assessed by fluorescence microscopy. Transferrin recycling and the number of TfRs present on the surface of the transfected cells were reduced by overexpression of either Rab11wt or the mutants. The amount of released exosomes was analyzed by measuring different molecular markers present on these vesicles either biochemically or by western blot. Overexpression of the dominant-negative mutant Rab11S25N inhibited exosome release, whereas the secretion of exosomes was slightly stimulated in cells transfected with Rab11wt. Taken together, the results demonstrate that in K562 cells Rab11 modulates the exosome pathway although the exact step involved is still not known.
Acetylcholinesterase↗
Targeting protein kinase C and "non-kinase" phorbol ester receptors: emerging concepts and therapeutic implications.
Phorbol esters, natural compounds that mimic the action of the lipid second messenger diacylglycerol (DAG), are known to exert their biological actions through the activation of classical and novel protein kinase C (PKC) isozymes. Phorbol esters, via binding to the PKC C1 domains, cause major effects on mitogenesis by controlling the activity of cyclin-cdk complexes and the expression of cdk inhibitors. In the last years it became clear that phorbol esters activate other molecules having a C1 domain in addition to PKCs. One of the most interesting families of "non-kinase" phorbol ester receptors is represented by the chimaerins, lipid-regulated Rac-GAPs that modulate actin cytoskeleton reorganization, migration, and proliferation. The discovery of the chimaerins and other "non-kinase" phorbol ester receptors has major implications in the design of agents for cancer therapy.
Animals↗