Co-localization of calcium-modulating cyclophilin ligand with intracellular calcium pools

Tail Anchored protein insertion mediated by CAML and TRC40 links to neuromuscular function in mice

Motor neuron diseases, such as amyotrophic lateral sclerosis (ALS) and progressive bulbar palsy, involve loss of muscle control resulting from death of motor neurons. Although the exact pathogenesis of these syndromes remains elusive, many are caused by genetically inherited mutations. Thus, it is valuable to identify additional genes that can impact motor neuron survival and function. In this report, we describe mice that express globally reduced levels of calcium-modulating cyclophilin ligand (CAML) protein. CAML is an essential component in the transmembrane domain recognition complex (TRC) pathway, responsible for inserting C-terminal tail anchored (TA) proteins into the endoplasmic reticulum membrane. The primary phenotype observed in these mice was rapid development of hind limb weakness and paralysis. Spinal cord sections revealed a loss of motor neuron cell bodies. Targeting CAML loss specifically to neurons using SLICK-H-Cre or synapsin-Cre transgenic mice yielded similar phenotypes, indicating that CAML plays a cell autonomous role in this process. We found that intracellular trafficking was perturbed in cells depleted of CAML, with aberrant release of procathepsin D and defective retention of CD222 within the trans-Golgi network, as well as reduced levels and mislocalization of syntaxin 5 (Stx5). Dysfunctional lysosomes and abnormal protein glycosylation were also revealed in CAML deficient cells, further indicating a defect in Golgi trafficking. In addition, we observed an identical phenotype in mice lacking ASNA1 in neurons, suggesting that CAML's role in sustaining muscle function is related to its involvement in the TRC pathway. Together, these findings implicate motor neuron survival as a key role for the TA protein insertion machinery in mice, which may shed light on the pathogenesis of neuromuscular disease in humans.

Proteomic analysis of Biliverdin protected cerebral ischemia-reperfusion injury in rats

Biliverdin, a heme metabolite, has been previously reported to alleviate cerebral ischemic reperfusion injury (CIRI). However, the alterations of brain proteome profiles underlying this treatment remain elusive. The objective of this study is to analyze the differential protein expression profile in cerebral cortex of rats involved in anti-CIRI effects of Biliverdin, providing experimental foundation for searching specific marker proteins. Rat model of MCAO/R was established, HE staining, TTC staining, TUNEL staining, and neurological behavioral examination, corner turning test, adhesive removal test, were performed to validate the effects of Biliverdin, and the results indicated that Biliverdin plays a significant role in alleviating CIRI. Furthermore, proteomic analysis of brain tissues of rats subjected to CIRI following Biliverdin treatment was performed using an integrated TMT-based quantitative proteomic approach coupled with LC-MS/MS technology to clarify the comprehensive mechanisms of Biliverdin in CIRI. First, we conducted strict quality control data for TMT experiments. Finally, a total of 7366 proteins were identified, of which 95 proteins were differentially expressed (DEPs) between the CIRI group and the Sham group and 52 between the CIRI and BV groups. In addition, two overlapping proteins among the 147 DEPs, Atg4c and Camlg, were validated by RT-qPCR and western blotting, and their levels were consistent with the results of TMT analysis. Taken together, the current findings firstly mapped comprehensive proteomic changes after CIRI treated with Biliverdin, providing a foundation for developing potentially therapeutic targets of anti-CIRI of Biliverdin and clinically prognostic biomarkers of stroke.

Preparation and Preliminary Application of Epitope Peptide-Based Antibody against Toxoplasma gondii GRA3

Toxoplasma gondii dense granule protein GRA3 has been shown to promote Toxoplasma gondii transmission and proliferation by interacting with the host cell endoplasmic reticulum (ER) through calcium-regulated cyclophilin ligands (CAMLG). Although many studies have focused on the interaction between the host cell endoplasmic reticulum and GRA3, no polyclonal antibodies (PcAbs) against GRA3 have been reported to date. According to the antigenicity prediction and exposure site analysis, three antigen peptide sequences were selected to prepare polyclonal antibodies targeting GRA3. Peptide scans revealed that the major antigenic epitope sequences were 125ELYDRTDRPGLK136, 202FFRRRPKDGGAG213, and 68NEAGESYSSATSG80, respectively. The GRA3 PcAb specifically recognized the GRA3 of T. gondii type Ⅱ ME49. The development of PcAbs against GRA3 is expected to elucidate the molecular mechanisms by which GRA3 regulates host cell function and contribute to the development of diagnostic and therapeutic strategies for toxoplasmosis.

Toxoplasma gondii dense granule protein 3 promotes endoplasmic reticulum stress-induced apoptosis by activating the PERK pathway

Background

Toxoplasma gondii is a neurotropic single-celled parasite that can infect mammals, including humans. Central nervous system infection with T. gondii infection can lead to Toxoplasma encephalitis. Toxoplasma infection can cause endoplasmic reticulum (ER) stress and unfolded protein response (UPR) activation, which ultimately can lead to apoptosis of host cells. The dense granule protein GRA3 has been identified as one of the secretory proteins that contribute to the virulence of T. gondii; however, the mechanism remains enigmatic.

Methods

The expression of the GRA3 gene in RH, ME49, Wh3, and Wh6 strains was determined using quantitative real-time polymerase chain reaction (qRT–PCR). pEGFP-GRA3_Wh6 was constructed by inserting Chinese 1 Wh6 GRA3 (GRA3_Wh6) cDNA into a plasmid encoding the enhanced GFP. Mouse neuro2a (N2a) cells were transfected with either pEGFP or pEGFP-GRA3_Wh6 (GRA3_Wh6) and incubated for 24–36 h. N2a cell apoptosis and ER stress-associated proteins were determined using flow cytometry and immunoblotting. Furthermore, N2a cells were pretreated with GSK2656157 (a PERK inhibitor) and Z-ATAD-FMK (a caspase-12 inhibitor) before GRA3_Wh6 transfection, and the effect of the inhibitors on GRA3_Wh6-induced ER stress and apoptosis were investigated.

Results

GRA3 gene expression was higher in the less virulent strains of type II ME49 and type Chinese 1 Wh6 strains compared with the virulent strains of type I RH strain and type Chinese 1 Wh3 strain. Transfection with GRA3_Wh6 plasmid induced neuronal apoptosis and increased the expression of GRP78, p-PERK, cleaved caspase-12, cleaved caspase-3, and CHOP compared with the control vector. Pretreatment with GSK2656157 and Z-ATAD-FMK decreased apoptosis in N2a cells, and similarly, ER stress- and apoptosis-associated protein levels were significantly decreased.

Conclusion

GRA3 induces neural cell apoptosis via the ER stress signaling pathway, which could play a role in toxoplasmic encephalitis.

Graphical Abstract

The Osteogenesis Imperfecta Type V Mutant BRIL/IFITM5 Promotes Transcriptional Activation of MEF2, NFATc, and NR4A in Osteoblasts

BRIL (bone restricted ifitm-like; also known as IFITM5) is a transmembrane protein expressed in osteoblasts. Although its role in skeletal development and homeostasis is unknown, mutations in BRIL result in rare dominant forms of osteogenesis imperfecta. The pathogenic mechanism has been proposed to be a gain-of or neomorphic function. To understand the function of BRIL and its OI type V mutant (MALEP BRIL) and whether they could activate signaling pathways in osteoblasts, we performed a luciferase reporter assay screen based on the activity of 26 transcription factors. When overexpressed in MC3T3-E1 and MLO-A5 cells, the MALEP BRIL activated the reporters dependent on MEF2, NFATc, and NR4A significantly more. Additional co-transfection experiments with MEF2C and NFATc1 and a number of their modulators (HDAC4, calcineurin, RCAN, FK506) confirmed the additive or synergistic activation of the pathways by MALEP, and suggested a coordinated regulation involving calcineurin. Endogenous levels of Nr4a members, as well as Ptgs2, were upregulated by MALEP BRIL. Y2H and co-immunoprecipitation indicated that BRIL interacted with CAML, but its contribution as the most upstream stimulator of the Ca²⁺-calcineurin-MEF2/NFATc cascade was not confirmed convincingly. Altogether the data presented provide the first ever readout to monitor for BRIL activity and suggest a potential gain-of-function causative effect for MALEP BRIL in OI type V, leading to perturbed signaling events and gene expression.

The WRB Subunit of the Get3 Receptor is Required for the Correct Integration of its Partner CAML into the ER

Calcium-modulating cyclophilin ligand (CAML), together with Tryptophan rich basic protein (WRB, Get1 in yeast), constitutes the mammalian receptor for the Transmembrane Recognition Complex subunit of 40 kDa (TRC40, Get3 in yeast), a cytosolic ATPase with a central role in the post-translational targeting pathway of tail-anchored (TA) proteins to the endoplasmic reticulum (ER) membrane. CAML has also been implicated in other cell-specific processes, notably in immune cell survival, and has been found in molar excess over WRB in different cell types. Notwithstanding the stoichiometric imbalance, WRB and CAML depend strictly on each other for expression. Here, we investigated the mechanism by which WRB impacts CAML levels. We demonstrate that CAML, generated in the presence of sufficient WRB levels, is inserted into the ER membrane with three transmembrane segments (TMs) in its C-terminal region. By contrast, without sufficient levels of WRB, CAML fails to adopt this topology, and is instead incompletely integrated to generate two aberrant topoforms; these congregate in ER-associated clusters and are degraded by the proteasome. Our results suggest that WRB, a member of the recently proposed Oxa1 superfamily, acts catalytically to assist the topogenesis of CAML and may have wider functions in membrane biogenesis than previously appreciated.

Modulation of intracellular calcium signaling by microRNA-34a-5p

Adjusting intracellular calcium signaling is an important feature in the regulation of immune cell function and survival. Here we show that miR-34a-5p, a small non-coding RNA that is deregulated in many common diseases, is a regulator of store-operated Ca²⁺ entry (SOCE) and calcineurin signaling. Upon miR-34a-5p overexpression, we observed both a decreased depletion of ER calcium content and a decreased Ca²⁺ influx through Ca²⁺ release-activated Ca²⁺ channels. Based on an in silico target prediction we identified multiple miR-34a-5p target genes within both pathways that are implicated in the balance between T-cell activation and apoptosis including ITPR2, CAMLG, STIM1, ORAI3, RCAN1, PPP3R1, and NFATC4. Functional analysis revealed a decrease in Ca²⁺ activated calcineurin pathway activity measured by a reduced IL-2 secretion due to miR-34a-5p overexpression. Impacting SOCE and/or downstream calcineurin/NFAT signaling by miR-34a-5p offers a possible future approach to manipulate immune cells for clinical interventions.

Classical Swine Fever Virus p7 Protein Interacts with Host Protein CAMLG and Regulates Calcium Permeability at the Endoplasmic Reticulum

We have previously shown that Classical Swine Fever Virus (CSFV) p7 is an essential nonstructural protein with a viroporin activity, a critical function in the progression of virus infection. We also identified p7 domains and amino acid residues critical for pore formation. Here, we describe how p7 specifically interacts with host protein CAMLG, an integral ER transmembrane protein involved in intracellular calcium release regulation and signal response generation. Detection of interaction as well as the identification of p7 areas mediating interaction with CAMLG was performed by yeast two-hybrid. p7-CAMLG interaction was further confirmed by confocal microscopy in eukaryotic cells, co-expressing both proteins. Mutant forms of p7 having substituted native residues identified as mediating interaction with CAMLG showed a decreased co-localization compared with the native forms of p7. Furthermore, it is shown that native p7, but not the mutated forms of p7 that fail to interact with CAMLG, efficiently mediates calcium permeability in the ER. Interestingly, viruses harboring some of those mutated forms of p7 have been previously shown to have a significantly decreased virulence in swine.

TMUB1 Inhibits BRL-3A Hepatocyte Proliferation by Interfering with the Binding of CAML to Cyclophilin B through its TM1 Hydrophobic Domain

Transmembrane and ubiquitin-like domain-containing 1 (Tmub1) encodes a protein (TMUB1) containing an ubiquitin-like domain and plays a negative regulatory role during hepatocyte proliferation, but its mechanism in this process is still unknown. Here, TMUB1 interfered with the binding of calcium-modulating cyclophilin ligand (CAML) to cyclophilin B, which may represent a key role in the negative regulatory process of TMUB1 in hepatocyte proliferation. Co-immunoprecipitation assays in rat BRL-3A cells confirmed the interaction between TMUB1 and CAML; significant regulation of the influx of Ca2+ ([Ca2+]i) and hepatocyte proliferation occurred following TMUB1 overexpression or knockout. Deletion of the TM1 hydrophobic domain of TMUB1 completely abolished this interaction and led to loss of TMUB1's regulatory effects on cytological behavior. Furthermore, overexpression of TMUB1 completely abolished the interaction between CAML and its downstream protein cyclophilin B, which can act upstream of calcineurin by increasing [Ca2+]i during cell proliferation. Taken together, our results indicate that TMUB1 regulates BRL-3A hepatocyte proliferation by interacting with CAML and further interferes with the binding of CAML to cyclophilin B to decrease cellular [Ca2+]i.

CAML mediates survival of Myc-induced lymphoma cells independent of tail-anchored protein insertion

Calcium-modulating cyclophilin ligand (CAML) is an endoplasmic reticulum (ER) protein that functions, along with WRB and TRC40, to mediate tail-anchored (TA) protein insertion into the ER membrane. Physiologic roles for CAML include endocytic trafficking, intracellular calcium signaling, and the survival and proliferation of specialized immune cells, recently attributed to its requirement for TA protein insertion. To identify a possible role for CAML in cancer cells, we generated Eμ-Myc transgenic mice that carry a tamoxifen-inducible deletion allele of Caml. In multiple B-cell lymphoma cell lines derived from these mice, homozygous loss of Caml activated apoptosis. Cell death was blocked by Bcl-2/Bcl-x_L overexpression; however, rescue from apoptosis was insufficient to restore proliferation. Tumors established from an Eμ-Myc lymphoma cell line completely regressed after tamoxifen administration, suggesting that CAML is also required for these cancer cells to survive and grow in vivo. Cell cycle analyses of Caml-deleted lymphoma cells revealed an arrest in G2/M, accompanied by low expression of the mitotic marker, phospho-histone H3 (Ser10). Surprisingly, lymphoma cell viability did not depend on the domain of CAML required for its interaction with TRC40. Furthermore, a small protein fragment consisting of the C-terminal 111 amino acid residues of CAML, encompassing the WRB-binding domain, was sufficient to rescue growth and survival of Caml-deleted lymphoma cells. Critically, this minimal region of CAML did not restore TA protein insertion in knockout cells. Taken together, these data reveal an essential role for CAML in supporting survival and mitotic progression in Myc-driven lymphomas that is independent of its TA protein insertion function.

Tail-anchored Protein Insertion in Mammals: FUNCTION AND RECIPROCAL INTERACTIONS OF THE TWO SUBUNITS OF THE TRC40 RECEPTOR

The GET (guided entry of tail-anchored proteins)/TRC (transmembrane recognition complex) pathway for tail-anchored protein targeting to the endoplasmic reticulum (ER) has been characterized in detail in yeast and is thought to function similarly in mammals, where the orthologue of the central ATPase, Get3, is known as TRC40 or Asna1. Get3/TRC40 function requires an ER receptor, which in yeast consists of the Get1/Get2 heterotetramer and in mammals of the WRB protein (tryptophan-rich basic protein), homologous to yeast Get1, in combination with CAML (calcium-modulating cyclophilin ligand), which is not homologous to Get2. To better characterize the mammalian receptor, we investigated the role of endogenous WRB and CAML in tail-anchored protein insertion as well as their association, concentration, and stoichiometry in rat liver microsomes and cultured cells. Functional proteoliposomes, reconstituted from a microsomal detergent extract, lost their activity when made with an extract depleted of TRC40-associated proteins or of CAML itself, whereas in vitro synthesized CAML and WRB together were sufficient to confer insertion competence to liposomes. CAML was found to be in ∼5-fold excess over WRB, and alteration of this ratio did not inhibit insertion. Depletion of each subunit affected the levels of the other one; in the case of CAML silencing, this effect was attributable to destabilization of the WRB transcript and not of WRB protein itself. These results reveal unanticipated complexity in the mutual regulation of the TRC40 receptor subunits and raise the question as to the role of the excess CAML in the mammalian ER.

Calcium-Modulating Cyclophilin Ligand Is Essential for the Survival of Activated T Cells and for Adaptive Immunity

Calcium-modulating cyclophilin ligand (CAML) is an endoplasmic reticulum resident protein that is widely expressed. Although it has been demonstrated to participate in the tail-anchored protein insertion pathway, its physiological role in the mature immune system is unknown. In this work, we show that mature, peripheral T cells require CAML for survival specifically following TCR-induced activation. In this study, we examined mature T cells from spleen and lymph nodes of tamoxifen-inducible CAML knockout mice (tCAML(-/-)). Whereas CAML-deficient T cells were able to express the early activation markers CD25 and CD69, and produce IL-2 normally upon stimulation, deficient cells proliferated less and died. Cells did not require CAML for entry into the S phase of the cell cycle, thus implicating its survival function at a relatively late step in the T cell activation sequence. In addition, CAML was required for homeostatic proliferation and for Ag-dependent cell killing in vivo. These results demonstrate that CAML critically supports T cell survival and cell division downstream of T cell activation.

The emerging role of calcium-modulating cyclophilin ligand in posttranslational insertion of tail-anchored proteins into the endoplasmic reticulum membrane

Tail-anchored (TA) proteins, a class of membrane proteins having an N-terminal cytoplasmic region anchored to the membrane by a single C-terminal transmembrane domain, are posttranslationally inserted into the endoplasmic reticulum (ER) membrane. In yeasts, the posttranslational membrane insertion is mediated by the Guided Entry of TA Proteins (GET) complex. Get3, a cytosolic ATPase, targets newly synthesized TA proteins to the ER membrane, where Get2 and Get3 constitute the Get3 receptor driving the membrane insertion. While mammalian cells employ TRC40 and WRB, mammalian homologs of Get3 and Get1, respectively, they lack the gene homologous to Get2. We recently identified calcium-modulating cyclophilin ligand (CAML) as a TRC40 receptor, indicating that CAML was equivalent to Get2 in the context of the membrane insertion. On the other hand, CAML has been well characterized as a signaling molecule that regulates various biological processes, raising the question of how the two distinct actions of CAML, the membrane insertion and the signal transduction, are assembled. In this review, we summarize recent progress of the molecular mechanism of the membrane insertion of TA proteins and discuss the possibility that CAML could sense the various signals at the ER membrane, thereby controlling TA protein biogenesis.

A novel interaction between calcium-modulating cyclophilin ligand and Basigin regulates calcium signaling and matrix metalloproteinase activities in human melanoma cells

Intracellular free calcium is a ubiquitous second messenger regulating a multitude of normal and pathogenic cellular responses, including the development of melanoma. Upstream signaling pathways regulating the intracellular free calcium concentration ([Ca2+]i) may therefore have a significant impact on melanoma growth and metastasis. In this study, we demonstrate that the endoplasmic reticulum (ER)-associated protein calcium-modulating cyclophilin ligand (CAML) is bound to Basigin, a widely expressed integral plasma membrane glycoprotein and extracellular matrix metalloproteinase inducer (EMMPRIN, or CD147) implicated in melanoma proliferation, invasiveness, and metastasis. This interaction between CAML and Basigin was first identified using yeast two-hybrid screening and further confirmed by co-immunoprecipitation. In human A375 melanoma cells, CAML and Basigin were co-localized to the ER. Knockdown of Basigin in melanoma cells by siRNA significantly decreased resting [Ca2+]i and the [Ca2+]i increase induced by the sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor thapsigargin (TG), indicating that the interaction between CAML and Basigin regulates ER-dependent [Ca2+]i signaling. Meanwhile upregulating the [Ca2+]i either by TG or phorbol myristate acetate (PMA) could stimulate the production of MMP-9 in A375 cells with the expression of Basigin. Our study has revealed a previously uncharacterized [Ca2+]i signaling pathway that may control melanoma invasion, and metastasis. Disruption of this pathway may be a novel therapeutic strategy for melanoma treatment.

Essential role for CAML in follicular B cell survival and homeostasis

Calcium-modulating cyclophilin ligand (CAML) is a ubiquitously expressed protein that is important during thymopoiesis. However, whether it serves a function in mature lymphocytes is unknown. In this article, we show that CAML is essential for survival of peripheral follicular (Fo) B cells. Conditional deletion of CAML in CD19-Cre transgenic mice caused a significant reduction in Fo cell numbers and increased rates of homeostatic proliferation. CAML-deficient Fo cells showed increased cellular turnover and normal proliferative ability. Although CAML-deficient Fo cells responded to AgR stimulation and to B cell activating factor, they displayed decreased survival and increased apoptosis following stimulation with LPS and IL-4 in vitro. Failure to survive was not due to aberrant B cell development in the absence of CAML, because induced deletion of the gene in mature cells resulted in a similar phenotype. These data establish an essential and ongoing role for CAML in the long-term survival of mature B cells.

Conditional deletion of calcium-modulating cyclophilin ligand causes deafness in mice

Calcium-modulating cyclophilin ligand (Caml) is a ubiquitously expressed cytoplasmic protein that is involved in multiple signaling and developmental pathways. An observation in our laboratory of a protein-protein interaction between Caml and the cytoplasmic region of Cadherin23 led us to speculate that Caml might be important in the inner ear and play a role in the development and/or function of hair cells. To address this question, we generated a mouse line in which Caml expression was eliminated in Atoh1-expressing cells of the inner ear upon administration of tamoxifen. Tamoxifen was administered immediately after birth to neonates to assess the effect of loss of Caml in the inner ear during postnatal development. Hearing in treated animals was tested by auditory brain stem response (ABR) analysis and cochlear pathology was evaluated by light microscopy. Lack of Caml expression in the inner ear leads to severe loss of cochlear hair cells and complete deafness. Elucidating the role of Caml in the inner ear will aid our understanding of the molecular pathways important for auditory development and function.

CAML regulates Bim-dependent thymocyte death

Appropriate control of apoptosis during T lymphocyte differentiation is critical for destruction of T cells bearing potentially autoreactive or useless immuno-receptors and for survival of those T cells bearing antigen receptors that may recognize foreign proteins. Despite the well-established importance of thymocyte survival, the exact signals regulating thymocyte apoptosis have not been fully elucidated. Here, we show that thymocytes lacking the endoplasmic reticulum protein calcium-modulating cyclophilin ligand (CAML) failed to undergo normal T-cell development and exhibited dramatically increased rates of apoptosis. In vitro, CAML-deficient thymocytes accumulated high levels of reactive oxygen species (ROS) and underwent abnormally accelerated death in response to several cytotoxic stimuli, including treatment with etoposide, cytokine deprivation, or Fas ligation. Although neither p53 deletion nor loss of Fas rescued the survival and continued development of CAML-deficient thymocytes, removal of the pro-apoptotic BH3-only Bcl-2 family member Bim significantly restored their survival. This work reveals CAML to be a critically important regulator of ROS- and Bim-dependent thymocyte death.

RNF122: a novel ubiquitin ligase associated with calcium-modulating cyclophilin ligand

Background

RNF122 is a recently discovered RING finger protein that is associated with HEK293T cell viability and is overexpressed in anaplastic thyroid cancer cells. RNF122 owns a RING finger domain in C terminus and transmembrane domain in N terminus. However, the biological mechanism underlying RNF122 action remains unknown.

Results

In this study, we characterized RNF122 both biochemically and intracellularly in order to gain an understanding of its biological role. RNF122 was identified as a new ubiquitin ligase that can ubiquitinate itself and undergoes degradation in a RING finger-dependent manner. From a yeast two-hybrid screen, we identified calcium-modulating cyclophilin ligand (CAML) as an RNF122-interacting protein. To examine the interaction between CAML and RNF122, we performed co-immunoprecipitation and colocalization experiments using intact cells. What is more, we found that CAML is not a substrate of ubiquitin ligase RNF122, but that, instead, it stabilizes RNF122.

Conclusions

RNF122 can be characterized as a C3H2C3-type RING finger-containing E3 ubiquitin ligase localized to the ER. RNF122 promotes its own degradation in a RING finger-and proteasome-dependent manner. RNF122 interacts with CAML, and its E3 ubiquitin ligase activity was noted to be dependent on the RING finger domain.

CAML does not modulate tetherin-mediated restriction of HIV-1 particle release

Background

Tetherin/BST-2 is a recently-identified potent restriction factor in human cells that restricts HIV particle release following particle formation and budding at the plasma membrane. Vpu counteracts tetherin's restriction of particle release in a manner that has not yet been fully defined. We recently identified calcium-modulating cyclophilin ligand (CAML) as a Vpu-interacting protein that also restricts particle release. We hypothesized that CAML may act to enhance tetherin-mediated restriction of particle release and thereby explain how two distinct factors could be responsible for Vpu-responsive restriction.

Methodology/Principal Findings

Endogenous levels of tetherin in human cells correlated well with their restriction pattern and responsiveness to Vpu, while levels of cellular CAML protein did not. Tetherin but not CAML was inducible by interferon in a wide variety of human cells. Stable depletion of human CAML in restrictive HeLa cells had no effect on cell surface levels of tetherin, and failed to relieve tetherin-mediated restriction. Stable depletion of tetherin from HeLa cells, in contrast, rendered HeLa cells permissive and Vpu-unresponsive. Tetherin but not CAML expression in permissive human cells rendered them restrictive and Vpu responsive. Depletion of CAML had no influence on cell surface levels of tetherin.

Conclusions/Significance

We conclude that tetherin restricts particle release and does not require CAML for this effect. Furthermore, these results do not support a major role for CAML in restricting HIV particle release in human cells.

Physical and functional interactions of cyclophilin B with neuronal actin and peroxiredoxin-1 are modified by oxidative stress

Presynaptic actin was identified as a new Torpedo cyclophilin B partner captured in pull-down experiments and by coimmunoprecipitation. The cyclophilin B-actin pull-down interaction was insensitive to the blockade of peptidyl cis/trans prolyl isomerase and calcineurin activities and to the latrunculin A- and jasplakinolide-mediated perturbation of F-actin polymerization. Conversely, it was reduced by ATP and stimulated by a low Cu(2+) treatment of synaptosomes and by acrolydan-conjugated cyclophilin B. This Cu(2+)-induced stress, in parallel, stimulates the formation of GSH adducts with cysteines of synaptosomal actin followed by its deglutathionylation and its dimerization in the presence of higher Cu(2+) concentrations. The reversibility of the thiol processing of actin occurred in the same range of Cu(2+) concentrations that mediated a stronger cyclophilin B-actin interaction, suggesting cyclophilin B participation in antioxidant processes. Among 2-Cys-peroxiredoxin isoforms, mainly peroxiredoxin-1 was found in cell bodies and nerve endings. Functionally, both Torpedo and human peroxiredoxin-1 were activated in vitro by Torpedo cyclophilin B. Moreover, cyclophilin B, like thioredoxins, maintained an H(2)O(2)-dependent peroxidase activity of peroxiredoxin-1 in the presence of dithiothreitol. Thus, the monocysteinic Torpedo cyclophilin B is able to sustain peroxiredoxin-1 activity and might be involved in the presynaptic defense against oxidative stress affecting G-actin posttranslational changes and its redox signaling in nerve ending compartments.

CAML loss causes anaphase failure and chromosome missegregation

Calcium modulating cyclophilin ligand (CAML) is a ubiquitously expressed cytoplasmic protein that is implicated in the EGFR and LCK signaling pathways and required for early embryonic and thymocyte development. To further define the critical biological functions of CAML at the cellular level, we generated CAML-deleted mouse embryonic fibroblasts (MEFs) using an in vitro Cre-loxP mediated conditional knockout system. We found that CAML(-/-) MEFs have severely impaired proliferation and a strong reduction of normal anaphases. The primary mitotic defect of CAML(-/-) MEFs is that duplicated chromosomes fail to segregate in anaphase, resulting in nuclear bisection by the cleavage furrow as cells decondense their DNA and exit mitosis, highly reminiscent of the "cut" phenotype in fission yeast. This phenotype is due to spindle dysfunction rather than inability to resolve physical connections between sister chromatids. Furthermore, CAML(-/-) MEFs display defects often seen in cells with mitotic checkpoint gene deficiencies, including lagging and misaligned chromosomes and chromatin bridges. Consistent with this, we found that CAML(-/-) MEFs have a modestly weakened spindle assembly checkpoint (SAC) and increased aneuploidy. Thus, our data identify CAML as a novel chromosomal instability gene and suggest that CAML protein acts as a key regulator of mitotic spindle function and a modulator of SAC maintenance.

Interactions of viral protein U (Vpu) with cellular factors

Viral protein U (Vpu) is encoded by one of several accessory genes of HIV-1. Like the accessory gene products Vif and Vpr, Vpu targets host proteins such as CD4 for degradation via the recruitment of cellular multi-subunit ubiquitin ligases. Vpu also forms ion channels in cellular membranes. Through one or both of these attributes, Vpu antagonizes host cell factors that restrict the release of progeny virions from infected cells. A key target of Vpu has recently been identified as the interferon-induced transmembrane protein BST-2/CD317 (tetherin), which restricts viral replication by retaining nascent virions on the cell surface. The counteraction of this host defense allows Vpu to be considered an antagonist of the innate immune response to viral infection.

Interaction between parasitophorous vacuolar membrane-associated GRA3 and calcium modulating ligand of host cell endoplasmic reticulum in the parasitism of Toxoplasma gondii

A monoclonal antibody against Toxoplasma gondii of Tg556 clone (Tg556) blotted a 29 kDa protein, which was localized in the dense granules of tachyzoites and secreted into the parasitophorous vacuolar membrane (PVM) after infection to host cells. A cDNA fragment encoding the protein was obtained by screening a T. gondii cDNA expression library with Tg556, and the full-length was completed by 5'-RACE of 2,086 bp containing an open reading frame (ORF) of 669 bp. The ORF encoded a polypeptide of 222 amino acids homologous to the revised GRA3 but not to the first reported one. The polypeptide has 3 hydrophobic moieties of an N-terminal stop transfer sequence and 2 transmembrane domains (TMD) in posterior half of the sequence, a cytoplasmic localization motif after the second TMD and an endoplasmic reticulum (ER) retrival motif in the C-terminal end, which suggests GRA3 as a type III transmembrane protein. With the ORF of GRA3, yeast two-hybrid assay was performed in HeLa cDNA expression library, which resulted in the interaction of GRA3 with calcium modulating ligand (CAMLG), a type II transmembrane protein of ER. The specific binding of GRA3 and CAMLG was confirmed by glutathione S-transferase (GST) pull-down and immunoprecipitation assays. The localities of fluorescence transfectionally expressed from GRA3 and CAMLG plasmids were overlapped completely in HeLa cell cytoplasm. In immunofluorescence assay, GRA3 and CAMLG were shown to be co-localized in the PVM of host cells. Structural binding of PVM-inserted GRA3 to CAMLG of ER suggested the receptor-ligand of ER recruitment to PVM during the parasitism of T. gondii.

Tumor necrosis factor (TNF)-alpha persistently activates nuclear factor-kappaB signaling through the type 2 TNF receptor in chromaffin cells: implications for long-term regulation of neuropeptide gene expression in inflammation

Chromaffin cells of the adrenal medulla elaborate and secrete catecholamines and neuropeptides for hormonal and paracrine signaling in stress and during inflammation. We have recently documented the action of the cytokine TNF-alpha on neuropeptide secretion and biosynthesis in isolated bovine chromaffin cells. Here, we demonstrate that the type 2 TNF-alpha receptor (TNF-R2) mediates TNF-alpha signaling in chromaffin cells via activation of nuclear factor (NF)-kappaB. Microarray and suppression subtractive hybridization have been used to identify TNF-alpha target genes in addition to those encoding the neuropeptides galanin, vasoactive intestinal polypeptide, and secretogranin II in chromaffin cells. TNF-alpha, acting through the TNF-R2, causes an early up-regulation of NF-kappaB, long-lasting induction of the NF-kappaB target gene inhibitor kappaB (IkappaB), and persistent stimulation of other NF-kappaB-associated genes including mitogen-inducible gene-6 (MIG-6), which acts as an IkappaB signaling antagonist, and butyrate-induced transcript 1. Consistent with long-term activation of the NF-kappaB signaling pathway, delayed induction of neuropeptide gene transcription by TNF-alpha in chromaffin cells is blocked by an antagonist of NF-kappaB signaling. TNF-alpha-dependent signaling in neuroendocrine cells thus leads to a unique, persistent mode of NF-kappaB activation that features long-lasting transcription of both IkappaB and MIG-6, which may play a role in the long-lasting effects of TNF-alpha in regulating neuropeptide output from the adrenal, a potentially important feedback station for modulating long-term cytokine effects in inflammation.

Calcium-modulating cyclophilin ligand regulates membrane trafficking of postsynaptic GABA(A) receptors

Accumulation of GABA(A) receptors (GABA(A)Rs) at GABAergic synapses requires the cytoplasmic loop region and C-terminal transmembrane domain of the receptor gamma2 subunit. We here report a novel interaction of gamma2 with Calcium-Modulating cyclophilin Ligand (CAML), an integral membrane protein that regulates this mechanism. Interaction of GABA(A)Rs with CAML depends on both the cytoplasmic region and fourth transmembrane domain of the gamma2 subunit, CAML immunoprecipitates with GABA(A)Rs from transfected cells and brain lysates and colocalizes with gamma2 in ER vesicles in soma and dendrites of neurons. CAML shRNA treatment results in reduced expression of postsynaptic GABA(A)Rs, along with significant reductions in GABA-evoked whole cell currents and GABAergic synaptic function, while glutamatergic transmission is unaffected. Reduced surface expression of GABA(A)Rs in CAML mutant neurons is associated with selective deficits in recycling of endocytosed GABA(A)Rs to the cell surface. Our results indicate a specific role of CAML in functional expression and endocytic recycling of postsynaptic GABA(A)Rs.

Identification of a novel immunosubversion mechanism mediated by a virologue of the B-lymphocyte receptor TACI

TACI (transmembrane activator and calcium modulator and cyclophilin ligand [CAML] interactor) is a part of a novel network of ligands and receptors involved in B-cell survival and isotype switching. The TACI protein mediates its effects through CAML, an endoplasmic reticulum (ER)-localized protein that controls Ca(2+) efflux. The adenovirus E3-6.7K protein prevents inflammatory responses and also confers resistance from a variety of apoptotic stimuli and maintains ER Ca(2+) homeostasis; however, the mechanism of action is unknown. Here, we provide evidence that E3-6.7K shares sequence homology with TACI and inhibits apoptosis and ER Ca(2+) efflux through an interaction with CAML, a Ca(2+)-modulating protein. We demonstrate a direct interaction between E3-6.7K and CAML and reveal that the two proteins colocalize in an ER-like compartment. Furthermore, the interaction between the two proteins is localized to the N-terminal domain of CAML and to a 22-amino-acid region near the C terminus of E3-6.7K termed the CAML-binding domain (CBD). Mutational analysis of the CBD showed that an interaction with CAML is required for E3-6.7K to inhibit thapsigargin-induced apoptosis and ER Ca(2+) efflux. E3-6.7K appears to be the first virologue of TACI to be identified. It targets CAML in a novel immunosubversive mechanism to alter ER Ca(2+) homeostasis, which consequently inhibits inflammation and protects infected cells from apoptosis.

Research progress in calcium-modulating cyclophilin ligand

Calcium-modulating cyclophilin ligand (CAML) is a ubiquitous protein that has been implicated in signal transduction, cell apoptosis, immune regulation and virus infection, although its role and mechanism of action are unknown. Many proteins, including transmembrane activator and CAML interactor, epidermal growth factor receptor, angiotensin 1 receptor-associated protein, immediate early gene X-1 protein, p56Lck, fibrocystin (coded by PKHD1), and Kaposi's sarcoma-associated herpesvirus protein, have been proved to be interacting with CAML. In this article, the recent development of CAML-associated study is reviewed.

Fibrocystin interacts with CAML, a protein involved in Ca2+ signaling

The predicted structure of the autosomal recessive polycystic kidney disease protein, fibrocystin, suggests that it may function as a receptor, but its function remains unknown. To understand its function, we searched for proteins that interact with the intracellular C-terminus of fibrocystin using the yeast two-hybrid system. From the screening, we found calcium modulating cyclophilin ligand (CAML), a protein involved in Ca(2+) signaling. Immunofluorescent analysis showed that both proteins are co-localized in the apical membrane, primary cilia, and the basal body of cells derived from the distal nephron Epitope-tagged expression constructs of both proteins were co-immunoprecipitated from COS7 cells. The intracellular C-terminus of fibrocystin interacts with CAML, a protein with an intracellular distribution that is similar to that of PKD2. Fibrocystin may participate in regulation of intracellular Ca(2+) in the distal nephron in a manner similar to PKD1 and PKD2 that are involved in autosomal dominant polycystic kidney disease.

CAML is a p56Lck-interacting protein that is required for thymocyte development

Calcium modulating cyclophilin ligand (CAML) is a ubiquitously expressed protein implicated in T cell signaling, although its mechanism and physiologic role in the immune system are unknown. We show here that CAML is essential for peripheral T cell development. Inactivation of CAML in mouse thymocytes lowered the numbers of double-positive and single-positive thymocytes, concomitant with reduced positive and enhanced negative selection. We found that CAML interacts with p56Lck and appears to regulate subcellular localization of the kinase in both resting and T cell receptor (TCR)-stimulated cells. CAML-deficient cells displayed enhanced p56lck and ZAP-70 phosphorylation and increased IL2 production and cell death after TCR stimulation, suggesting that CAML may act as a negative regulator of p56lck. Our data establish a novel role for CAML as an essential mediator of T cell survival during thymopoiesis and indicate that its loss deregulates p56Lck signaling.

Identification of calcium-modulating cyclophilin ligand (CAML) as transducer of angiotensin II-mediated nuclear factor of activated T cells (NFAT) activation

Angiotensin II (Ang II) plays a central role in cardiovascular physiology and disease. Ang II type I receptor (AT1) is thought to mediate most actions of Ang II. A novel AT1 receptor intracellular partner called AT1 receptor-associated protein (ATRAP) was identified, but its exact function has not been elucidated. A yeast two-hybrid screen using ATRAP as bait identified calcium-modulating cyclophilin ligand (CAML) as an ATRAP partner. Yeast two-hybrid and coimmunoprecipitation analysis demonstrated that the N-terminal hydrophilic domain of CAML (amino acids (aa) 1-189) mediates a specific interaction between ATRAP and CAML. Our analysis also showed that aa 40-82 of ATRAP contribute to this interaction. Bioluminescence resonance energy transfer and intracellular colocalization analysis by immunofluorescence in HEK293 cells verified this association within endoplasmic reticulum vesicular structures. Functionally, transcriptional reporter assays and RNA interference ATRAP experiments demonstrated that ATRAP knockdown increased nuclear factor of activated T cells (NFAT) activity. Overexpression of ATRAP decreased Ang II- or CAML-induced NFAT transcriptional activation, whereas an ATRAP-interacting domain of CAML (aa 1-189) sensitized NFAT activation in response to Ang II. These results indicate that CAML is an important signal transducer for the actions of Ang II in regulating the calcineurin-NFAT pathway and suggest that the interaction of CAML with ATRAP may mediate the Ang II actions in vascular physiology.

Simultaneous gene expression analysis of steady-state and actively translated mRNA populations from osteosarcoma MG-63 cells in response to IL-1alpha via an open expression analysis platform

Pro-inflammatory cytokines play a key role in various forms of metabolic bone diseases, including osteopenia and osteoporosis. Human MG-63 cells treated with IL-1alpha were used as a model system to identify potential marker genes that are differentially expressed. This study is designed to quantitate gene expression of actively translated mRNAs as compared to the steady-state mRNA population. Both steady-state mRNAs and actively translated mRNAs from control MG-63 cells and MG-63 cells treated with IL-1alpha were isolated and converted to cDNA. The gene expression analysis from these samples was then quantitated with an open expression analysis platform with no requirement for a priori knowledge of sequence information. As a result, many differentially regulated genes were discovered via IL-1alpha treatment. Some of the genes have been described previously as playing important roles in the regulation of inflammation and cell adhesion. These comparisons provided a panoramic overview of gene expression at both the total transcript and post-transcriptional levels. In addition, the quantitation of actively translated mRNAs associated with polysomes also provided a better estimation of protein expression levels. This methodology allows for the identification of genes acutely regulated during translation. Furthermore, the process may aid in the identification of new drug targets or biomarkers.

CAML is required for efficient EGF receptor recycling

Calcium-modulating cyclophilin ligand (CAML) is a ubiquitous protein that has been implicated in signaling from the cell surface receptor TACI in lymphocytes, although its role and mechanism of action are unknown. To study its function in the mouse, we disrupted the CAML gene and found it to be required for early embryonic development, but not for cellular viability. CAML-deficient cells have severely impaired proliferative responses to the epidermal growth factor (EGF). Although EGF-induced activation of signaling intermediates and internalization of the EGF receptor (EGFR) are normal in the absence of CAML, the recycling of internalized receptors to the plasma membrane is defective, leading to its reduced surface accumulation. We demonstrate that CAML normally associates directly with the kinase domain of the EGFR in a ligand-dependent manner. These data implicate CAML in EGFR signaling and suggest that it may play a role in receptor recycling during long-term proliferative responses to EGF.

A conserved calcineurin-binding motif in human T lymphotropic virus type 1 p12I functions to modulate nuclear factor of activated T cell activation

The PXIXIT calcineurin binding motif or highly related sequences are found in a variety of calcineurin-binding proteins in yeast, mammalian cells, and viruses. The accessory protein p12(I) encoded in the HTLV-1 pX ORF I promotes T cell activation during the early stages of HTLV-1 infection by activating nuclear factor of activated T cells (NFAT) through calcium release from the endoplasmic reticulum. We identified in p12(I), a conserved motif, which is highly homologous with the PXIXIT calcineurin-binding motif of NFAT. Both immunoprecipitation and calmodulin agarose bead pull-down assays indicated that wild type p12(I) and mutants of p12(I) that contained the motif-bound calcineurin. In addition, an alanine substitution p12(I) mutant (p12(I) AXAXAA) had greatly reduced binding affinity for calcineurin. We then tested whether p12(I) binding to calcineurin affected NFAT activity. p12(I) competed with NFAT for calcineurin binding in calmodulin bead pull-down experiments. Furthermore, the p12(I) AXAXAA mutant enhanced NFAT nuclear translocation compared with wild type p12(I) and increased NFAT transcriptional activity 2-fold greater than wild type p12(I). Similar to NFAT, endogenous calcineurin phosphatase activity was increased in Jurkat T cells expressing p12(I) independent of its calcineurin binding property. Thus, the reduced binding of p12(I) to calcineurin allows enhanced nuclear translocation and transcription mediated by NFAT. Herein, we are the first to identify a retroviral protein that binds calcineurin. Our data suggest that HTLV-1 p12(I) modulates NFAT activation to promote early virus infection of T lymphocytes, providing a novel mechanism for retrovirus-mediated cell activation.

Critical role of human T-lymphotropic virus type 1 accessory proteins in viral replication and pathogenesis

Human T-cell lymphotropic virus type 1 (HTLV-1) infection is associated with a diverse range of lymphoproliferative and neurodegenerative diseases, yet pathogenic mechanisms induced by the virus remain obscure. This complex retrovirus contains typical structural and enzymatic genes but also unique regulatory and accessory genes in four open reading frames (ORFs) of the pX region of the viral genome (pX ORFs I to IV). The regulatory proteins encoded by pX ORFs III and IV, Tax and Rex, respectively, have been extensively characterized. In contrast the contribution of the four accessory proteins p12(I), p27(I), p13(II), and p30(II), encoded by pX ORFs I and II, to viral replication and pathogenesis remained unclear. Proviral clones that are mutated in either pX ORF I or II, while fully competent in cell culture, are severely limited in their replicative capacity in a rabbit model. Emerging evidence indicates that the HTLV-1 accessory proteins are critical for establishment of viral infectivity, enhance T-lymphocyte activation, and potentially alter gene transcription and mitochondrial function. HTLV-1 pX ORF I expression is critical to the viral infectivity in resting primary lymphocytes, suggesting a role for p12(I) in lymphocyte activation. The endoplasmic reticulum and cis-Golgi localizing p12(I), encoded from pX ORF I, activates NFAT, a key T-cell transcription factor, through calcium-mediated signaling pathways and may lower the threshold of lymphocyte activation via the JAK/STAT pathway. In contrast p30(II) localizes to the nucleus and represses viral promoter activity, but may regulate cellular gene expression through p300/CBP or related coactivators of transcription. p13(II) targets mitochondrial proteins, where it alters the organelle morphology and may influence energy metabolism. Collectively, studies of the molecular functions of the HTLV-1 accessory proteins provide insight into strategies used by retroviruses that are associated with lymphoproliferative diseases.

Cloning of rat calcium-modulating cyclophilin ligand

Rat calcium-modulating cyclophilin ligand (CAML) cDNA was cloned and sequenced. It has a predicted open reading frame of 294 amino acids. The CAML gene is highly conserved throughout species, showing 85, 89 and 69% amino acid sequence identity to the human, mouse, and chicken genes, respectively. Gene expression data using astrocytes, microglia and neurons show that CAML mRNA and protein is constitutively expressed in these cell types of the central nervous system. The cloning of rat CAML will facilitate further investigations on the function of this molecule.

Activation of nuclear factor of activated T cells by human T-lymphotropic virus type 1 accessory protein p12(I)

Human T-lymphotropic virus type 1 (HTLV-1) is the agent of an aggressive malignancy of CD4(+) T lymphocytes, called adult T-cell lymphoma/leukemia, and is associated with numerous immune-mediated diseases. To establish infection, HTLV-1 must activate targeted T cells during early stages of infection. We recently demonstrated that the HTLV-1 accessory protein p12(I) is critical for persistent infection in vivo and for viral infectivity in quiescent primary lymphocytes, suggesting a role for p12(I) in lymphocyte activation. To test whether p12(I) modulates signaling pathways required for T-lymphocyte activation, we examined AP-1-, NF-kappaB-, and nuclear factor of activated T cells (NFAT)-driven reporter gene activity in p12(I)-expressing Jurkat T cells compared to vector-transfected control cells. HTLV-1 p12(I) specifically induced NFAT-mediated transcription approximately 20-fold in synergy with the Ras/mitogen-activated protein kinase pathway, but did not influence AP-1- or NF-kappaB-dependent gene expression. Inhibition of calcium-dependent signals by cyclosporin A, BAPTA-AM [glycine, N,N'-1,2-ethanediylbis(oxy-2,1-phenylene)-bis-N-2-(acetyloxy)methoxy-2-oxoethyl]-[bis(acetyloxy)methyl ester], and a dominant negative mutant of NFAT2 abolished the p12(I)-mediated activation of NFAT-dependent transcription. In contrast, inhibition of phospholipase C-gamma and LAT (linker for activation of T cells) did not affect p12(I)-induced NFAT activity. Importantly, p12(I) functionally substituted for thapsigargin, which selectively depletes intracellular calcium stores. Our data are the first to demonstrate a role for HTLV-1 p12(I) in calcium-dependent activation of NFAT-mediated transcription in lymphoid cells. We propose a novel mechanism by which HTLV-1, a virus associated with lymphoproliferative disease, dysregulates common T-cell activation pathways critical for the virus to establish persistent infection.

The transcriptional signature of dioxin in human hepatoma HepG2 cells

We have used a high density microarray hybridization approach to characterize the transcriptional response of human hepatoma HepG2 cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We find that exposure to 10 nM TCDD for 8 hr alters by at least a factor of 2.1 the expression of 310 known genes and of an equivalent number of expressed sequence tags. Treatment with TCDD in the presence of 20 microg/mL of cycloheximide blocked the effect on 202 of these genes, allowing us to distinguish between primary effects of TCDD exposure, which take place whether cycloheximide is present or not, and secondary effects, which are blocked by inhibition of protein synthesis. Of the 310 known genes affected by TCDD, 30 are up-regulated and 78 are down-regulated regardless of cycloheximide treatment, and 84 are up-regulated and 118 are down-regulated only when protein synthesis is not inhibited. Functional clustering of genes regulated by TCDD reveals many potential physiological interactions that might shed light on the multiple biological effects of this compound. Our results, however, suggest that arriving at a sound understanding of the molecular mechanisms governing the biological outcome of TCDD exposure promises to be orders of magnitude more complicated than might have been previously imagined.

The mitochondrial permeability transition pore and its role in cell death

This article reviews the involvement of the mitochondrial permeability transition pore in necrotic and apoptotic cell death. The pore is formed from a complex of the voltage-dependent anion channel (VDAC), the adenine nucleotide translocase and cyclophilin-D (CyP-D) at contact sites between the mitochondrial outer and inner membranes. In vitro, under pseudopathological conditions of oxidative stress, relatively high Ca2+ and low ATP, the complex flickers into an open-pore state allowing free diffusion of low-Mr solutes across the inner membrane. These conditions correspond to those that unfold during tissue ischaemia and reperfusion, suggesting that pore opening may be an important factor in the pathogenesis of necrotic cell death following ischaemia/reperfusion. Evidence that the pore does open during ischaemia/reperfusion is discussed. There are also strong indications that the VDAC-adenine nucleotide translocase-CyP-D complex can recruit a number of other proteins, including Bax, and that the complex is utilized in some capacity during apoptosis. The apoptotic pathway is amplified by the release of apoptogenic proteins from the mitochondrial intermembrane space, including cytochrome c, apoptosis-inducing factor and some procaspases. Current evidence that the pore complex is involved in outer-membrane rupture and release of these proteins during programmed cell death is reviewed, along with indications that transient pore opening may provoke 'accidental' apoptosis.

Identification of simian cyclophilin A as a calreticulin-binding protein in yeast two-hybrid screen and demonstration of cyclophilin A interaction with calreticulin

Cyclophilin A (CyPA) was identified as one of the calreticulin (CR)-binding proteins in a yeast two-hybrid screen utilizing simian cDNA expression-library. The simian CyPA protein had 96% identity with that of human, differing only at eight amino acid residues. We further established CyPA-CR interaction by incubation of glutathione transferase-fused CyPA (GST-CyPA) and CR proteins with CV-1 cyto-lysates, followed by CR and CyPA-specific immuno-blot analysis. The immunosuppressive drug cyclosporin A, a CyPA ligand, did not inhibit CyPA-CR interaction. Our results established a new property of CyPA binding activity to CR. Since CR is a Ca2+-binding protein, CR-CyPA interactions may be important in signaling pathways for induction of Ca2+-dependent cellular processes.