Apoptosis-linked gene product ALG-2 is a new member of the calpain small subunit subfamily of Ca2+-binding proteins

Ca2+-sensor ALG-2 engages ESCRTs to enhance lysosomal membrane resilience to osmotic stress

Lysosomes are central players in cellular catabolism, signaling, and metabolic regulation. Cellular and environmental stresses that damage lysosomal membranes can compromise their function and release toxic content into the cytoplasm. Here, we examine how cells respond to osmotic stress within lysosomes. Using sensitive assays of lysosomal leakage and rupture, we examine acute effects of the osmotic disruptant glycyl-L-phenylalanine 2-naphthylamide (GPN). Our findings reveal that low concentrations of GPN rupture a small fraction of lysosomes, but surprisingly trigger Ca2+ release from nearly all. Chelating cytoplasmic Ca2+ makes lysosomes more sensitive to GPN-induced rupture, suggesting a role for Ca2+ in lysosomal membrane resilience. GPN-elicited Ca2+ release causes the Ca2+-sensor Apoptosis Linked Gene-2 (ALG-2), along with Endosomal Sorting Complex Required for Transport (ESCRT) proteins it interacts with, to redistribute onto lysosomes. Functionally, ALG-2, but not its ESCRT binding-disabled ΔGF122 splice variant, increases lysosomal resilience to osmotic stress. Importantly, elevating juxta-lysosomal Ca2+ without membrane damage by activating TRPML1 also recruits ALG-2 and ESCRTs, protecting lysosomes from subsequent osmotic rupture. These findings reveal that Ca2+, through ALG-2, helps bring ESCRTs to lysosomes to enhance their resilience and maintain organelle integrity in the face of osmotic stress.

Multifaceted roles of PDCD6 both within and outside the cell

Programmed cell death protein 6 (PDCD6) is an evolutionarily conserved Ca2+-binding protein. PDCD6 is involved in regulating multifaceted and pleiotropic cellular processes in different cellular compartments. For instance, nuclear PDCD6 regulates apoptosis and alternative splicing. PDCD6 is required for coat protein complex II-dependent endoplasmic reticulum-to-Golgi apparatus vesicular transport in the cytoplasm. Recent advances suggest that cytoplasmic PDCD6 is involved in the regulation of cytoskeletal dynamics and innate immune responses. Additionally, membranous PDCD6 participates in membrane repair through endosomal sorting complex required for transport complex-dependent membrane budding. Interestingly, extracellular vesicles are rich in PDCD6. Moreover, abnormal expression of PDCD6 is closely associated with many diseases, especially cancer. PDCD6 is therefore a multifaceted but pivotal protein in vivo. To gain a more comprehensive understanding of PDCD6 functions and to focus and stimulate PDCD6 research, this review summarizes key developments in its role in different subcellular compartments, processes, and pathologies.

Ca 2+ -sensor ALG-2 engages ESCRTs to enhance lysosomal membrane resilience to osmotic stress

Lysosomes are central players in cellular catabolism, signaling, and metabolic regulation. Cellular and environmental stresses that damage lysosomal membranes can compromise their function and release toxic content into the cytoplasm. Here, we examine how cells respond to osmotic stress within lysosomes. Using sensitive assays of lysosomal leakage and rupture, we examine acute effects of the cathepsin C-metabolized osmotic disruptant glycyl-L-phenylalanine 2-naphthylamide (GPN). Our findings reveal that widely used concentrations of GPN rupture only a small fraction of lysosomes, but surprisingly trigger Ca 2+ release from nearly all. Chelating cytoplasmic Ca 2+ using BAPTA makes lysosomes more likely to rupture under GPN-induced stress, suggesting that Ca 2+ plays a role in protecting or rapidly repairing lysosomal membranes. Mechanistically, we establish that GPN causes the Ca 2+ -sensitive protein Apoptosis Linked Gene-2 (ALG-2) and interacting ESCRT proteins to redistribute onto lysosomes, improving their resistance to membrane stress created by GPN as well as the lysosomotropic drug chlorpromazine. Furthermore, we show that activating the cation channel TRPML1, with or without blocking the endoplasmic reticulum Ca 2+ pump, creates local Ca 2+ signals that protect lysosomes from rupture by recruiting ALG-2 and ESCRTs without any membrane damage. These findings reveal that Ca 2+ , through ALG-2, helps bring ESCRTs to lysosomes to enhance their resilience and maintain organelle integrity in the face of osmotic stress.

SIGNIFICANCE

As the degradative hub of the cell, lysosomes are full of toxic content that can spill into the cytoplasm. There has been much recent interest in how cells sense and repair lysosomal membrane damage using ESCRTs and cholesterol to rapidly fix "nanoscale damage". Here, we extend understanding of how ESCRTs contribute by uncovering a preventative role of the ESCRT machinery. We show that ESCRTs, when recruited by the Ca 2+ -sensor ALG-2, play a critical role in stabilizing the lysosomal membrane against osmotically-induced rupture. This finding suggests that cells have mechanisms not just for repairing but also for actively protecting lysosomes from stress-induced membrane damage.

Role of calcium-sensor proteins in cell membrane repair

Cell membrane repair is a critical process used to maintain cell integrity and survival from potentially lethal chemical, and mechanical membrane injury. Rapid increases in local calcium levels due to a membrane rupture have been widely accepted as a trigger for multiple membrane-resealing models that utilize exocytosis, endocytosis, patching, and shedding mechanisms. Calcium-sensor proteins, such as synaptotagmins (Syt), dysferlin, S100 proteins, and annexins, have all been identified to regulate, or participate in, multiple modes of membrane repair. Dysfunction of membrane repair from inefficiencies or genetic alterations in these proteins contributes to diseases such as muscular dystrophy (MD) and heart disease. The present review covers the role of some of the key calcium-sensor proteins and their involvement in membrane repair.

In vitro reconstitution of calcium-dependent recruitment of the human ESCRT machinery in lysosomal membrane repair

The endosomal sorting complex required for transport (ESCRT) machinery is centrally involved in the repair of damage to both the plasma and lysosome membranes. ESCRT recruitment to sites of damage occurs on a fast time scale, and Ca2+ has been proposed to play a key signaling role in the process. Here, we show that the Ca2+-binding regulatory protein ALG-2 binds directly to negatively charged membranes in a Ca2+-dependent manner. Next, by monitoring the colocalization of ALIX with ALG-2 on negatively charged membranes, we show that ALG-2 recruits ALIX to the membrane. Furthermore, we show that ALIX recruitment to the membrane orchestrates the downstream assembly of late-acting CHMP4B, CHMP3, and CHMP2A subunits along with the AAA+ ATPase VPS4B. Finally, we show that ALG-2 can also recruit the ESCRT-III machinery to the membrane via the canonical ESCRT-I/II pathway. Our reconstitution experiments delineate the minimal sets of components needed to assemble the entire membrane repair machinery and open an avenue for the mechanistic understanding of endolysosomal membrane repair.

Interplay Between Non-Coding RNAs and Programmed Cell Death Proteins

Programmed cell death (PDCD) family of proteins includes at least 12 members, function of seven of them being more investigated. These members are PDCD1, PDCD2, PDCD4, PDCD5, PDCD6, PDCD7 and PDCD10. Consistent with the important roles of these proteins in the regulation of apoptosis, dysregulation of PDCDs is associated with diverse disorders ranging from intervertebral disc degeneration, amyotrophic lateral sclerosis, immune thrombocytopenia, type 1 diabetes, congenital hypothyroidism, Alzheimer’s disease to different types of cancers. More recently, the interaction between non-coding RNAs and different members of PDCD family is being discovered. In the current study, we described the functional interactions between PDCDs and two classes of non-coding RNAs, namely microRNAs (miRNAs) and long non-coding RNAs (lncRNAs). miR-21 and miR-183 are two miRNAs whose interactions with PDCDs have been assessed in different contexts. The lncRNAs interaction with PDCDs is mainly assessed in the context of neoplasia indicating the role of MALAT1, MEG3, SNHG14 and LINC00473 in this process.

Lysosomal TRPML1 Channel: Implications in Cardiovascular and Kidney Diseases

Lysosomal ion channels mediate ion flux from lysosomes and regulate membrane potential across the lysosomal membrane, which are essential for lysosome biogenesis, nutrient sensing, lysosome trafficking, lysosome enzyme activity, and cell membrane repair. As a cation channel, the transient receptor potential mucolipin 1 (TRPML1) channel is mainly expressed on lysosomes and late endosomes. Recently, the normal function of TRPML1 channels has been demonstrated to be important for the maintenance of cardiovascular and renal glomerular homeostasis and thereby involved in the pathogenesis of some cardiovascular and kidney diseases. In arterial myocytes, it has been found that Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP), an intracellular second messenger, can induce Ca²⁺ release through the lysosomal TRPML1 channel, leading to a global Ca²⁺ release response from the sarcoplasmic reticulum (SR). In podocytes, it has been demonstrated that lysosomal TRPML1 channels control lysosome trafficking and exosome release, which contribute to the maintenance of podocyte functional integrity. The defect or functional deficiency of lysosomal TRPML1 channels has been shown to critically contribute to the initiation and development of some chronic degeneration or diseases in the cardiovascular system or kidneys. Here we briefly summarize the current evidence demonstrating the regulation of lysosomal TRPML1 channel activity and related signaling mechanisms. We also provide some insights into the canonical and noncanonical roles of TRPML1 channel dysfunction as a potential pathogenic mechanism for certain cardiovascular and kidney diseases and associated therapeutic strategies.

The Penta-EF-Hand ALG-2 Protein Interacts with the Cytosolic Domain of the SOCE Regulator SARAF and Interferes with Ubiquitination

ALG-2 is a penta-EF-hand Ca²⁺-binding protein and interacts with a variety of proteins in mammalian cells. In order to find new ALG-2-binding partners, we searched a human protein database and retrieved sequences containing the previously identified ALG-2-binding motif type 2 (ABM-2). After selecting 12 high-scored sequences, we expressed partial or full-length GFP-fused proteins in HEK293 cells and performed a semi-quantitative in vitro binding assay. SARAF, a negative regulator of store-operated Ca²⁺ entry (SOCE), showed the strongest binding activity. Biochemical analysis of Strep-tagged and GFP-fused SARAF proteins revealed ubiquitination that proceeded during pulldown assays under certain buffer conditions. Overexpression of ALG-2 interfered with ubiquitination of wild-type SARAF but not ubiquitination of the F228S mutant that had impaired ALG-2-binding activity. The SARAF cytosolic domain (CytD) contains two PPXY motifs targeted by the WW domains of NEDD4 family E3 ubiquitin ligases. The PPXY motif proximal to the ABM-2 sequence was found to be more important for both in-cell ubiquitination and post-cell lysis ubiquitination. A ubiquitination-defective mutant of SARAF with Lys-to-Arg substitutions in the CytD showed a slower degradation rate by half-life analysis. ALG-2 promoted Ca²⁺-dependent CytD-to-CytD interactions of SARAF. The ALG-2 dimer may modulate the stability of SARAF by sterically blocking ubiquitination and by bridging SARAF molecules at the CytDs.

ALG-2 couples T cell activation and apoptosis by regulating proteasome activity and influencing MCL1 stability

T cell homeostasis is critical for the proper function of the immune system. Following the sharp expansion upon pathogen infection, most T cells die in order to keep balance in the immune system, a process which is controlled by death receptors during the early phase and Bcl-2 proteins in the later phase. It is still highly debated whether the apoptosis of T cells is determined from the beginning, upon activation, or determined later during the contraction. MCL1, a Bcl-2 family member, plays a pivotal role in T cell survival. As a fast turnover protein, MCL1 levels are tightly regulated by the 26S proteasome-controlled protein degradation process. In searching for regulatory factors involved in the actions of MCL1 during T cell apoptosis, we found that ALG-2 was critical for MCL1 stability, a process mediated by a direct interaction between ALG-2 and Rpn3, a key component of the 26S proteasome. As a critical calcium sensor, ALG-2 regulated the activity of the 26S proteasome upon increases to cytosolic calcium levels following T cell activation, this consequently influenced the stability of MCL1 and accelerated the T cell “death” process, leading to T cell contraction and restoration of immune homeostasis. Our study provides support for the notion that T cells are destined for apoptosis after activation, and echoes the previous study about the function of ALG-2 in T cell death.

The Molecular Evolution and Functional Divergence of Lamprey Programmed Cell Death Genes

The programmed cell death (PDCD) family plays a significant role in the regulation of cell survival and apoptotic cell death. However, the evolution, distribution and role of the PDCD family in lampreys have not been revealed. Thus, we identified the PDCD gene family in the lamprey genome and classified the genes into five subfamilies based on orthologs of the genes, conserved synteny, functional domains, phylogenetic tree, and conserved motifs. The distribution of the lamprey PDCD family and the immune response of the PDCD family in lampreys stimulated by different pathogens were also demonstrated. In addition, we investigated the molecular function of lamprey PDCD2, PDCD5, and PDCD10. Our studies showed that the recombinant lamprey PDCD5 protein and transfection of the L-PDCD5 gene induced cell apoptosis, upregulated the expression of the associated X protein (BAX) and TP53 and downregulated the expression of B cell lymphoma 2 (BCL-2) independent of Caspase 3. In contrast, lamprey PDCD10 suppressed apoptosis in response to cis-diaminedichloro-platinum (II) stimuli. Our phylogenetic and functional data not only provide a better understanding of the evolution of lamprey PDCD genes but also reveal the conservation of PDCD genes in apoptosis. Overall, our results provide a novel perspective on lamprey immune regulation mediated by the PDCD family.

Thermodynamic Characterization of the Ca2+-Dependent Interaction Between SOUL and ALG-2

SOUL, a heme-binding protein-2 (HEBP-2), interacts with apoptosis-linked gene 2 protein (ALG-2) in a Ca²⁺-dependent manner. To investigate the properties of the interaction of SOUL with ALG-2, we generated several mutants of SOUL and ALG-2 and analyzed the recombinant proteins using pulldown assay and isothermal titration calorimetry. The interaction between SOUL and ALG-2 (delta3-23ALG-2) was an exothermic reaction, with 1:1 stoichiometry and high affinity (Kd = 32.4 nM) in the presence of Ca²⁺. The heat capacity change (ΔCp) of the reaction showed a large negative value (−390 cal/K·mol), which suggested the burial of a significant nonpolar surface area or disruption of a hydrogen bond network that was induced by the interaction (or both). One-point mutation of SOUL Phe100 or ALG-2 Trp57 resulted in complete loss of heat change, supporting the essential roles of these residues for the interaction. Nevertheless, a truncated mutant of SOUL1-143 that deleted the domain required for the interaction with ALG-2 Trp57 still showed 1:1 binding to ALG-2 with an endothermic reaction. These results provide a better understanding of the target recognition mechanism and conformational change of SOUL in the interaction with ALG-2.

Molecular Characterisation and Functions of Fis1 and PDCD6 Genes from Echinococcus granulosus

Cystic echinococcosis, a parasitic zoonosis that causes significant economic losses and poses a threat to public health, is caused by larvae of the tapeworm Echinococcus granulosus. Infection causes infertile cysts in intermediate hosts that cannot produce protoscoleces (PSCs) or complete the life cycle. Herein, we cloned, expressed, and characterised mitochondrial fission protein 1 (Eg-Fis1) and programmed cell death protein 6 (Eg-PDCD6) from E. granulosus, and explored their functions related to infertile cysts. Eg-Fis1 and Eg-PDCD6 encode putative 157 and 174 residue proteins, respectively, and Western blotting indicated good reactogenicity for both. Eg-Fis1 and Eg-PDCD6 were ubiquitously distributed in all stages of E. granulosus. Furthermore, mRNAs of Eg-Fis1 and Eg-PDCD6 were upregulated following H₂O₂ treatment which induced apoptosis in PSCs. To investigate the regulation of apoptosis in response to oxidative stress, RNA interference (RNAi) and terminal deoxynucleotidyl transferase dUTP nick-end labelling (TUNEL) assays were performed. The apoptotic rate of the Eg-Fis1 RNAi group was significantly lower than non-interference group, but there was no such difference for Eg-PDCD6. In conclusion, Eg-Fis1 promotes apoptosis induced by oxidative stress, whereas Eg-PDCD6 does not appear to be a key regulator of apoptosis.

Nanoluciferase Reporter Gene System Directed by Tandemly Repeated Pseudo-Palindromic NFAT-Response Elements Facilitates Analysis of Biological Endpoint Effects of Cellular Ca2+ Mobilization

NFAT is a cytoplasm-localized hyper-phosphorylated transcription factor that is activated through dephosphorylation by calcineurin, a Ca²⁺/calmodulin-dependent phosphatase. A non-palindromic NFAT-response element (RE) found in the IL2 promoter region has been commonly used for a Ca²⁺-response reporter gene system, but requirement of concomitant activation of AP-1 (Fos/Jun) often complicates the interpretation of obtained results. A new nanoluciferase (NanoLuc) reporter gene containing nine-tandem repeats of a pseudo-palindromic NFAT-RE located upstream of the IL8 promoter was designed to monitor Ca²⁺-induced transactivation activity of NFAT in human embryonic kidney (HEK) 293 cells by measuring luciferase activities of NanoLuc and co-expressed firefly luciferase for normalization. Ionomycin treatment enhanced the relative luciferase activity (RLA), which was suppressed by calcineurin inhibitors. HEK293 cells that stably express human STIM1 and Orai1, components of the store-operated calcium entry (SOCE) machinery, gave a much higher RLA by stimulation with thapsigargin, an inhibitor of sarcoplasmic/endoplamic reticulum Ca²⁺-ATPase (SERCA). HEK293 cells deficient in a penta-EF-hand Ca²⁺-binding protein ALG-2 showed a higher RLA value than the parental cells by stimulation with an acetylcholine receptor agonist carbachol. The novel reporter gene system is found to be useful for applications to cell signaling research to monitor biological endpoint effects of cellular Ca²⁺ mobilization.

The calcium-binding protein ALG-2 regulates protein secretion and trafficking via interactions with MISSL and MAP1B proteins

Mobilization of intracellular calcium is essential for a wide range of cellular processes, including signal transduction, apoptosis, and vesicular trafficking. Several lines of evidence have suggested that apoptosis-linked gene 2 (ALG-2, also known as PDCD6), a calcium-binding protein, acts as a calcium sensor linking calcium levels with efficient vesicular trafficking, especially at the endoplasmic reticulum (ER)-to-Golgi transport step. However, how ALG-2 regulates these processes remains largely unclear. Here, we report that MAPK1-interacting and spindle-stabilizing (MISS)-like (MISSL), a previously uncharacterized protein, interacts with ALG-2 in a calcium-dependent manner. Live-cell imaging revealed that upon a rise in intracellular calcium levels, GFP-tagged MISSL (GFP-MISSL) dynamically relocalizes in a punctate pattern and colocalizes with ALG-2. MISSL knockdown caused disorganization of the components of the ER exit site, the ER-Golgi intermediate compartment, and Golgi. Importantly, knockdown of either MISSL or ALG-2 attenuated the secretion of secreted alkaline phosphatase (SEAP), a model secreted cargo protein, with similar reductions in secretion by single- and double-protein knockdowns, suggesting that MISSL and ALG-2 act in the same pathway to regulate the secretion process. Furthermore, ALG-2 or MISSL knockdown delayed ER-to-Golgi transport of procollagen type I. We also found that ALG-2 and MISSL interact with microtubule-associated protein 1B (MAP1B) and that MAP1B knockdown reverts the reduced secretion of SEAP caused by MISSL or ALG-2 depletion. These results suggest that a change in the intracellular calcium level plays a role in regulation of the secretory pathway via interaction of ALG-2 with MISSL and MAP1B.

ALG2 regulates glioblastoma cell proliferation, migration and tumorigenicity

Apoptosis-linked gene-2 (ALG-2), also known as programmed cell death 6 (PDCD6), has recently been reported to be aberrantly expressed in various tumors and required for tumor cell viability. The aim of the present study was to investigate whether ALG-2 plays a crucial role in tumor cell proliferation, migration and tumorigenicity. In this study, we examined the expression of PDCD6 in glioblastoma cell lines and found that ALG-2 was generally expressed in glioblastoma cell lines. We also performed an analysis of an online database and found that high expression of ALG-2 was associated with poor prognosis (p = 0.039). We found that over-expression of ALG2 in glioblastoma could inhibit cell proliferation and, conversely, that down-regulation of ALG2 could promote cell proliferation. Further studies showed that over-expression of ALG2 inhibited the migration of tumor cells, whereas down-regulation of ALG2 promoted tumor cell migration. Finally, in vitro and in vivo studies showed that over-expression of ALG2 inhibited the tumorigenic ability of tumor cells, while down-regulation of ALG2 promoted tumor cell tumorigenic ability. In conclusion, ALG2 has a tumor suppressive role in glioblastoma and might be a potential target for the treatment of glioblastoma.

The calcium-binding protein ALG-2 promotes endoplasmic reticulum exit site localization and polymerization of Trk-fused gene (TFG) protein

Apoptosis-linked gene 2 (ALG-2), which is a gene product of PDCD6, is a 22-kDa Ca2+ -binding protein. Accumulating evidence points to a role for ALG-2 as a Ca2+ -responsive adaptor protein. On binding to Ca2+ , ALG-2 undergoes a conformational change that facilitates its interaction with various proteins. It also forms a homodimer and heterodimer with peflin, a paralog of ALG-2. However, the differences in cellular roles for the ALG-2 homodimer and ALG-2/peflin heterodimer are unclear. In the present study, we found that Trk-fused gene (TFG) protein interacted with the ALG-2 homodimer. Immunostaining analysis revealed that TFG and ALG-2 partially overlapped at endoplasmic reticulum exit sites (ERES), a platform for COPII-mediated protein transport from the endoplasmic reticulum. Time-lapse live-cell imaging demonstrated that both green fluorescent protein-fused TFG and mCherry-fused ALG-2 are recruited to ERES after thapsigargin treatment, which raises intracellular Ca2+ levels. Furthermore, overexpression of ALG-2 induced the accumulation of TFG at ERES. TFG has an ALG-2-binding motif and deletion of the motif decreased TFG binding to ALG-2 and shortened its half-life at ERES, suggesting a critical role for ALG-2 in retaining TFG at ERES. We also demonstrated, by in vitro cross-linking assays, that ALG-2 promoted the polymerization of TFG in a Ca2+ -dependent manner. Collectively, the results suggest that ALG-2 acts as a Ca2+ -sensitive adaptor to concentrate and polymerize TFG at ERES, supporting a potential role for ALG-2 in COPII-dependent trafficking from the endoplasmic reticulum.

Multifaceted Roles of ALG-2 in Ca(2+)-Regulated Membrane Trafficking

ALG-2 (gene name: PDCD6) is a penta-EF-hand Ca²⁺-binding protein and interacts with a variety of proteins in a Ca²⁺-dependent fashion. ALG-2 recognizes different types of identified motifs in Pro-rich regions by using different hydrophobic pockets, but other unknown modes of binding are also used for non-Pro-rich proteins. Most ALG-2-interacting proteins associate directly or indirectly with the plasma membrane or organelle membranes involving the endosomal sorting complex required for transport (ESCRT) system, coat protein complex II (COPII)-dependent ER-to-Golgi vesicular transport, and signal transduction from membrane receptors to downstream players. Binding of ALG-2 to targets may induce conformational change of the proteins. The ALG-2 dimer may also function as a Ca²⁺-dependent adaptor to bridge different partners and connect the subnetwork of interacting proteins.

Single nucleotide polymorphisms in PDCD6 gene are associated with the development of cervical squamous cell carcinoma

The programmed cell death 6 (PDCD6), discovered as a proapoptotic calcium-binding protein, has recently been found dysregulated in tumors of various origin and contributed to cancer cell viability. The aim of this study was to determine whether SNPs in PDCD6 are associated with cervical squamous cell carcinoma (CSCC). Polymerase chain reaction-restriction fragment length polymorphism method was used to genotype two tag SNPs (rs3756712 and rs4957014) of PDCD6 in 328 CSCC patients and 541 controls. Significantly increased CSCC risks were found to be associated with T allele of rs3756712 and G allele of rs4957014 (P = 0.017, OR = 1.320, and P = 0.007, OR = 1.321, respectively). CSCC risks were associated with these two SNPs in different genetic model (P = 0.04, OR = 1.78 for rs3756712 in a recessive model, and P = 0.006, OR = 2.01 for rs4957014 in a codominant model, respectively). Results of stratified analyses revealed that rs4957014 is associated with parametrial invasion of CSCC (P = 0.044, OR = 1.414). Our results suggest that these two tag SNPs of PDCD6 are associated with CSCC, indicating that PDCD6 may play an important role in the pathogenesis of CSCC.

Calcium signaling in membrane repair

Resealing allows cells to mend damaged membranes rapidly when plasma membrane (PM) disruptions occur. Models of PM repair mechanisms include the "lipid-patch", "endocytic removal", and "macro-vesicle shedding" models, all of which postulate a dependence on local increases in intracellular Ca(2+) at injury sites. Multiple calcium sensors, including synaptotagmin (Syt) VII, dysferlin, and apoptosis-linked gene-2 (ALG-2), are involved in PM resealing, suggesting that Ca(2+) may regulate multiple steps of the repair process. Although earlier studies focused exclusively on external Ca(2+), recent studies suggest that Ca(2+) release from intracellular stores may also be important for PM resealing. Hence, depending on injury size and the type of injury, multiple sources of Ca(2+) may be recruited to trigger and orchestrate repair processes. In this review, we discuss the mechanisms by which the resealing process is promoted by vesicular Ca(2+) channels and Ca(2+) sensors that accumulate at damage sites.

Molecular cloning of the apoptosis-related calcium-binding protein AsALG-2 in Avena sativa

Victorin, the host-selective toxin produced by the fungus Cochliobolus victoriae, induces programmed cell death (PCD) in victorin-sensitive oat lines with characteristic features of animal apoptosis, such as mitochondrial permeability transition, chromatin condensation, nuclear DNA laddering and rRNA/mRNA degradation. In this study, we characterized a calcium-binding protein, namely AsALG-2, which might have a role in the victorin-induced PCD. AsALG-2 is homologous to the Apoptosis-Linked Gene ALG-2 identified in mammalian cells. Northern blot analysis revealed that the accumulation of AsALG-2 transcripts increased during victorin-induced PCD, but not during necrotic cell death. Salicylic acid, chitosan and chitin strongly activated the expression of general defence response genes, such as PR-10; however, neither induced cell death nor the accumulation of AsALG-2 mRNA. Pharmacological studies indicated that victorin-induced DNA laddering and AsALG-2 expression were regulated through similar pathways. The calcium channel blocker, nifedipine, moderately inhibited the accumulation of AsALG-2 mRNA during cell death. Trifluoperazine (calmodulin antagonist) and K252a (serine-threonine kinase inhibitor) reduced the victorin-induced phytoalexin accumulation, but did not prevent the victorin-induced DNA laddering or accumulation of AsALG-2 mRNA. Taken together, our investigations suggest that there is a calcium-mediated signalling pathway in animal and plant PCD in common.

Association between two single nucleotide polymorphisms of PDCD6 gene and increased endometriosis risk

Programmed cell death 6 (PDCD6), a calcium binding protein of the penta EF-hand protein family, and its receptors are involved in regulation of apoptosis pathways. To evaluate the relationship between genetic polymorphisms of PDCD6 gene and endometriosis (ED) risk, we investigated the association of two single nucleotide polymorphisms (SNPs) of PDCD6 gene (rs4957014 and rs3756712) in 220 endometriosis patients and 386 unrelated healthy controls. The genotypes of these two SNPs were determined by using polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) and DNA sequencing methods. Significantly increased endometriosis risk was observed to be associated with G allele of rs4957014 locus (OR=1.31, 95% CI=1.03-1.69). We have also observed increased ED risk was statistically associated with rs4957014 polymorphism in a dominant model (OR=1.52, 95% CI=1.09-2.13). Although no association has been found between ED risk and the allele frequencies of rs3756712 locus (a marginal P=0.066, OR=1.27, 95% CI=0.98-1.65), but in a dominant model, increased endometriosis risk was significantly associated with rs3756712 polymorphism (OR=1.54, 95% CI=1.11-2.17). In conclusion, the current study indicates that PDCD6 gene may be a new susceptibility gene to endometriosis.

Programmed cell death 6 (PDCD6) as a prognostic marker for gastric cancers

Programmed cell death 6 (PDCD6) plays an important role in apoptotic cell death and tumorigenesis. In this study, we investigated whether PDCD6 contributes to the development and/or progression of gastric cancers. PDCD6 protein expression was examined in 169 advanced gastric cancer specimens by immunohistochemistry and then correlated with clinicopathologic parameters. We also analyzed mutations, methylation status, and alterations in DNA copy number and mRNA transcripts, and protein expression of PDCD6 in gastric cancers. The effect of PDCD6 on cell viability and death was further examined in wild- and mutant-type PDCD6 transfected AGS and HEK293T cell lines. Increased expression of PDCD6 expression was detected in 124 (73.4%) out of 169 gastric cancer specimens. Statistically, altered expression of PDCD6 was closely associated with survival rates (P = 0.0069). One non-sense mutation was found at codon 175 of PDCD6, and no hypermethylation was found in gastric cancers. Decreased copy numbers and mRNA expression of PDCD6 were found in 7 (16.7%) and 10 (23.8%) of 42 gastric cancer specimens, respectively. AGS and HEK293T cells transfected with wild-type PDCD6 showed marked inhibition of cell viability and induction of cell death via activation of mitochondrial cell death pathways, whereas mutant-type PDCD6 showed partial ablation of tumor suppressor activity. In addition, AGS cells transfected with wild-type PDCD6 and treated with 5-FU showed synergistic inhibition of cell viability (P < 0.001). These data provide evidence that the PDCD6 gene is a significant prognostic biomarker for advanced gastric cancer patients.

Structure and function of ALG-2, a penta-EF-hand calcium-dependent adaptor protein

ALG-2 (a gene product of PDCD6) is a 22-kD protein containing five serially repetitive EF-hand structures and belongs to the penta-EF-hand (PEF) family, including the subunits of typical calpains. ALG-2 is the most conserved protein among the PEF family members and its homologs are widely found in eukaryotes. X-ray crystal structures of various PEF proteins including ALG-2 have common features: presence of eight α-helices and dimer formation via paired EF5s that are positioned in anti-parallel orientation. ALG-2 forms a homodimer and a heterodimer with its closest paralog peflin. Like calmodulin, a well-known four-EF-hand protein, ALG-2 interacts with various proteins in a Ca(2+)-dependent fashion, but the binding motifs are completely different. With some exceptions, ALG-2-interacting proteins commonly contain Pro-rich regions, and ALG-2 recognizes at least two distinct Pro-containing motifs: PPYP(X)nYP (X, variable; n=4 in ALIX and PLSCR3) and PXPGF (represented by Sec31A). A shorter alternatively spliced isoform, lacking two residues and designated ALG-2(ΔGF122), does not bind ALIX but maintains binding capacity to Sec31A. X-ray crystal structural analyses have revealed that binding of calcium ions induces the configuration of the side chain of R125 so that it opens Pocket 1, which accepts PPYP, but Pocket 1 remains closed in the case of ALG-2(ΔGF122). ALG-2 dimer has two ligand-binding sites, each in a monomer molecule, and appears to function as a Ca(2+)-dependent adaptor protein to either stabilize a preformed complex or to bridge two proteins on scaffolds in systems of the endosomal sorting complex required for transport (ESCRT) and ER-to-Golgi transport.

Vesicular calcium regulates coat retention, fusogenicity, and size of pre-Golgi intermediates

This study establishes a role for luminal Ca²⁺ in ER/Golgi transport organelles and elucidates an effector mechanism involving the EF-hand protein ALG-2 and regulation of COPII coat retention.

The significance and extent of Ca²⁺ regulation of the biosynthetic secretory pathway have been difficult to establish, and our knowledge of regulatory relationships integrating Ca²⁺ with vesicle coats and function is rudimentary. Here, we investigated potential roles and mechanisms of luminal Ca²⁺ in the early secretory pathway. Specific depletion of luminal Ca²⁺ in living normal rat kidney cells using cyclopiazonic acid (CPA) resulted in the extreme expansion of vesicular tubular cluster (VTC) elements. Consistent with this, a suppressive role for vesicle-associated Ca²⁺ in COPII vesicle homotypic fusion was demonstrated in vitro using Ca²⁺ chelators. The EF-hand–containing protein apoptosis-linked gene 2 (ALG-2), previously implicated in the stabilization of sec31 at endoplasmic reticulum exit sites, inhibited COPII vesicle fusion in a Ca²⁺-requiring manner, suggesting that ALG-2 may be a sensor for the effects of vesicular Ca²⁺ on homotypic fusion. Immunoisolation established that Ca²⁺ chelation inhibits and ALG-2 specifically favors residual retention of the COPII outer shell protein sec31 on pre-Golgi fusion intermediates. We conclude that vesicle-associated Ca²⁺, acting through ALG-2, favors the retention of residual coat molecules that seem to suppress membrane fusion. We propose that in cells, these Ca²⁺-dependent mechanisms temporally regulate COPII vesicle interactions, VTC biogenesis, cargo sorting, and VTC maturation.

Identification of the penta-EF-hand protein ALG-2 as a Ca2+-dependent interactor of mucolipin-1

Loss of function mutations in mucolipin-1 (MCOLN1) have been linked to mucolipidosis type IV (MLIV), a recessive lysosomal storage disease characterized by severe neurological and ophthalmological abnormalities. MCOLN1 is an ion channel that regulates membrane transport along the endolysosomal pathway. It has been suggested that MCOLN1 participates in several Ca(2+)-dependent processes, including fusion of lysosomes with the plasma membrane, fusion of late endosomes and autophagosomes with lysosomes, and lysosomal biogenesis. Here, we searched for proteins that interact with MCOLN1 in a Ca(2+)-dependent manner. We found that the penta-EF-hand protein ALG-2 binds to the NH-terminal cytosolic tail of MCOLN1. The interaction is direct, strictly dependent on Ca(2+), and mediated by a patch of charged and hydrophobic residues located between MCOLN1 residues 37 and 49. We further show that MCOLN1 and ALG-2 co-localize to enlarged endosomes induced by overexpression of an ATPase-defective dominant-negative form of Vps4B (Vps4B(E235Q)). In agreement with the proposed role of MCOLN1 in the regulation of fusion/fission events, we found that overexpression of MCOLN1 caused accumulation of enlarged, aberrant endosomes that contain both early and late endosome markers. Interestingly, aggregation of abnormal endosomes was greatly reduced when the ALG-2-binding domain in MCOLN1 was mutated, suggesting that ALG-2 regulates MCOLN1 function. Overall, our data provide new insight into the molecular mechanisms that regulate MCOLN1 activity. We propose that ALG-2 acts as a Ca(2+) sensor that modulates the function of MCOLN1 along the late endosomal-lysosomal pathway.

Structural basis for Ca2+ -dependent formation of ALG-2/Alix peptide complex: Ca2+/EF3-driven arginine switch mechanism

ALG-2 belongs to the penta-EF-hand (PEF) protein family and interacts with various intracellular proteins, such as Alix and TSG101, that are involved in endosomal sorting and HIV budding. Through X-ray crystallography, we solved the structures of Ca(2+)-free and -bound forms of N-terminally truncated human ALG-2 (des3-20ALG-2), Zn(2+)-bound form of full-length ALG-2, and the structure of the complex between des3-23ALG-2 and the peptide corresponding to Alix799-814 in Zn(2+)-bound form. Binding of Ca(2+) to EF3 enables the side chain of Arg125, present in the loop connecting EF3 and EF4, to move enough to make a primary hydrophobic pocket accessible to the critical PPYP motif, which partially overlaps with the GPP motif for the binding of Cep55 (centrosome protein 55 kDa). Based on these results, together with the results of in vitro binding assay with mutant ALG-2 and Alix proteins, we propose a Ca(2+)/EF3-driven arginine switch mechanism for ALG-2 binding to Alix.

Caleosin of Arabidopsis thaliana : Effect of Calcium on Functional and Structural Properties

A non-radioactive blot binding assay has proved the capacity of a purified recombinant form of Arabidopsis thaliana caleosin (AtClo1), a key protein of this plant oil body, to bind calcium. Calcium affected recombinant caleosin aggregation state, solubility, and electrophoretic mobility on SDS-PAGE. The effect of calcium on interfacial behavior of recombinant caleosin was studied at three interfaces: air/water (A/W), purified oil/water (O/W), and air/phosholipid/water (A/PLs/W). Recombinant caleosin was able to decrease interfacial tension (IFT) at A/W and O/W interfaces as a function of concentration and calcium, whereas no interaction was detected at the A/PLs/W interface. Effect of calcium was time dependent, and its amplitude strongly varied with the interface considered. Reconstituted oil bodies were used to prove the involvement of recombinant caleosin in their calcium-driven aggregation and coalescence. Calcium ions at concentration as low as 100 nM were able to strongly modify the shape and aggregation state of purified oil bodies, as well as their behavior within a monolayer, reflecting potentially profound changes in their structure and dynamic.

Crystallization and X-ray diffraction analysis of N-terminally truncated human ALG-2

ALG-2 (apoptosis-linked gene 2) is an apoptosis-linked calcium-binding protein with five EF-hand motifs in the C-terminal region. N-terminally truncated ALG-2 (des3-23ALG-2) was crystallized by the vapour-diffusion method in buffer consisting of either 50 mM MES pH 6.5, 12.5%(v/v) 2-propanol and 150 mM calcium acetate or 100 mM MES pH 6.0, 15%(v/v) ethanol and 200 mM zinc acetate. Crystals of the Ca(2+)-bound form belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 54.8, b = 154.4, c = 237.7 A, alpha = beta = gamma = 90 degrees , and diffracted to 3.1 A resolution. Crystals of the Zn(2+)-bound form belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 52.8, b = 147.5, c = 230.7 A, alpha = beta = gamma = 90 degrees , and diffracted to 3.3 A resolution. The structures of the Ca(2+)-bound form and the Zn(2+)-bound form were solved by the molecular-replacement method. Although both crystals contained eight ALG-2 molecules per asymmetric unit, the metal-ion locations and octameric arrangements were found to be significantly different.

Identification of Alix-type and Non-Alix-type ALG-2-binding sites in human phospholipid scramblase 3: differential binding to an alternatively spliced isoform and amino acid-substituted mutants

ALG-2, a prototypic member of the penta-EF-hand protein family, interacts with Alix at its C-terminal Pro-rich region containing four tandem PXY repeats. Human phospholipid scramblase 3 (PLSCR3) has a similar sequence (ABS-1) in its N-terminal region. In the present study, we found that ALG-2 interacts with PLSCR3 expressed in HEK293 cells in a Ca(2+)-dependent manner by co-immunoprecipitation, pulldown with glutathione S-transferase (GST) fused ALG-2 and an overlay assay using biotin-labeled ALG-2. The GST fusion protein of an alternatively spliced isoform of ALG-2, GST-ALG-2(DeltaGF122), pulled down green fluorescent protein (GFP)-fused PLSCR3 but not GFP Alix. Deletion of a region containing ABS-1 was not sufficient to abrogate the binding. A second ALG-2-binding site (ABS-2) was essential for interaction with ALG-2(DeltaGF122). Real-time interaction analyses with a surface plasmon resonance biosensor using synthetic oligopeptides and recombinant proteins corroborated direct Ca(2+)-dependent binding of ABS-1 to ALG-2 and that of ABS-2 to ALG-2 as well as to ALG-2(DeltaGF122). The sequence of ABS-2 contains multiple prolines and two phenylalanines, among which Phe(49) was found to be critical, because its substitution with Ala or Tyr caused a loss of binding ability by pulldown assays using oligopeptide-immobilized beads. ALG-2-interacting proteins were classified into two groups based on binding ability to ALG-2(DeltaGF122): (i) isoform-non-interactive (ABS-1) types, including Alix, annexin A7, annexin A11, and TSG101 and (ii) isoform-interactive (ABS-2) types including PLSCR3, PLSCR4 and Sec31A. GST-pulldown assays using single amino acid-substituted ALG-2 mutants revealed differences in binding specificities between the two groups, suggesting structural flexibility in ALG-2-ligand complex formation.

The yeast penta-EF protein Pef1p is involved in cation-dependent budding and cell polarization

Penta-EF-hand (PEF) proteins bind calcium and participate in a variety of calcium-dependent processes in vertebrates. In yeast, intracellular cations regulate processes like cell division and polarized growth. This study reports the identification of a unique PEF protein in Saccharomyces cerevisiae encoded by the uncharacterized open reading frame YGR058w. Pef1p has a long and unstructured N-terminal domain conserved in ascomycetes, and a highly conserved C-terminal calcium binding domain homologous to human ALG-2 and sorcin. Pef1p binds calcium and zinc and homodimerizes in vitro and in vivo like vertebrate homologues. Disruption of PEF1 induces defective growth in SDS and cation depletion conditions. Significantly, a critical substitution in the second EF hand (E218A) lowers the in vitro affinity for zinc and phenocopies growth defects. The dissection of protein-protein interactions and the cellular localization of Pef1p analogous to that of RAM pathway components controlling daughter-specific gene expression at the site of bud emergence bring out the importance of this novel protein. Our data suggest that cation homeostasis is involved in the control of polarized growth and in stress response in budding yeast.

ALG-2 oscillates in subcellular localization, unitemporally with calcium oscillations

A variety of stimuli can trigger intracellular calcium oscillations. Relatively little is known about the molecular mechanisms decoding these events. We show that ALG-2, a Ca2+-binding protein originally isolated as a protein associated with apoptosis, is directly linked to Ca2+ signalling. We discovered that the subcellular distribution of a tagged version of ALG-2 could be directed by physiological external stimuli (including ATP, EGF, prostaglandin, histamine), which provoke intracellular Ca2+ oscillations. Cellular stimulation led to a redistribution of ALG-2 from the cytosol to a punctate localization in an oscillatory fashion unitemporally with Ca2+ oscillations, whereas a Ca2+-binding deficient mutant of ALG-2 did not redistribute. Using tagged ALG-2 as bait we identified its novel target protein Sec31A and based on the partial colocalization of endogenous ALG-2 and Sec31A we propose that ALG-2 temporarily binds to the COPII vesicles providing a link between Ca2+ signalling and ER to Golgi trafficking.

ALG-2 directly binds Sec31A and localizes at endoplasmic reticulum exit sites in a Ca2+-dependent manner

Intracellular localization of the penta-EF-hand Ca2+-binding protein ALG-2 in HeLa cells was investigated by immunofluorescent confocal microscopy using a polyclonal antibody. In addition to its presence in the nucleus, ALG-2 was found to be distributed in a punctate pattern in the cytoplasm, where it was partly co-stained with an endoplasmic reticulum (ER) exit site marker p125. In vitro GST pull down analysis demonstrated that ALG-2 and its alternatively spliced isoform interact with the COPII component Sec31A in a Ca2+-dependent manner, and a biotin-labeled ALG-2 overlay assay revealed direct binding of ALG-2 to Sec31A. Biochemical and immunofluorescent microscopic analyses showed that ALG-2 was enriched at the Sec31A-localizing membrane compartments upon stimulation with the Ca2+ ionophore A23187. In contrast, treatment of cells with the membrane-permeant Ca2+ chelator BAPTA-AM led to a dispersion of ALG-2 throughout the cells and to a significant loss of Sec31A in the perinuclear region. These findings establish Sec31A as a novel target for ALG-2 and provide a framework for studies on the roles of ALG-2 in ER-Golgi transport.

The Ca2+-binding protein ALG-2 is recruited to endoplasmic reticulum exit sites by Sec31A and stabilizes the localization of Sec31A

The formation of transport vesicles that bud from endoplasmic reticulum (ER) exit sites is dependent on the COPII coat made up of three components: the small GTPase Sar1, the Sec23/24 complex, and the Sec13/31 complex. Here, we provide evidence that apoptosis-linked gene 2 (ALG-2), a Ca(2+)-binding protein of unknown function, regulates the COPII function at ER exit sites in mammalian cells. ALG-2 bound to the Pro-rich region of Sec31A, a ubiquitously expressed mammalian orthologue of yeast Sec31, in a Ca(2+)-dependent manner and colocalized with Sec31A at ER exit sites. A Ca(2+) binding-deficient ALG-2 mutant, which did not bind Sec31A, lost the ability to localize to ER exit sites. Overexpression of the Pro-rich region of Sec31A or RNA interference-mediated Sec31A depletion also abolished the ALG-2 localization at these sites. In contrast, depletion of ALG-2 substantially reduced the level of Sec31A associated with the membrane at ER exit sites. Finally, treatment with a cell-permeable Ca(2+) chelator caused the mislocalization of ALG-2, which was accompanied by a reduced level of Sec31A at ER exit sites. We conclude that ALG-2 is recruited to ER exit sites via Ca(2+)-dependent interaction with Sec31A and in turn stabilizes the localization of Sec31A at these sites.

Proteomic approach in the search of new cardiovascular biomarkers

With the increasing incidence of cardiovascular diseases worldwide, specifically atherosclerosis and heart failure, the search for novel biomarkers remains a priority. As opposed to complex diagnostic techniques that may not be suitable to be applied to the wider population, biomarkers are useful for population screening. The search for novel biomarkers is based on knowledge of the molecular and cellular processes that take place in the development of a specific disease. Atherosclerosis and heart failure are characterized by a long period of silent disease progression, allowing early diagnosis and the potential of early therapeutic intervention. The use of the so-called proteomic techniques allows not only protein identification but partial characterization, which includes expression and also post-translational modification of these proteins. This allows for the discovery of previously unknown proteins involved in cardiovascular diseases, including some that may be suitable to be used as biomarkers. However, to approach this issue, we have to overcome difficulties such as tissue heterogeneity (vessel wall or myocardium) and the lack of fresh human samples. We discuss the proteomic study of human plaques, secreted proteins by pathologic and normal vessel wall, and left ventricular hypertrophy as potential sources of new biologic markers of cardiovascular disease.

The penta-EF-hand protein ALG-2 interacts directly with the ESCRT-I component TSG101, and Ca2+-dependently co-localizes to aberrant endosomes with dominant-negative AAA ATPase SKD1/Vps4B

ALG-2 (apoptosis-linked gene 2) is a Ca2+-binding protein that belongs to the PEF (penta-EF-hand) protein family. Alix (ALG-2-interacting protein X)/AIP1 (ALG-2-interacting protein 1), one of its binding partners, interacts with TSG101 and CHMP4 (charged multivesicular body protein 4), which are components of ESCRT-I (endosomal sorting complex required for transport I) and ESCRT-III respectively. In the present study, we investigated the association between ALG-2 and ESCRT-I. By a GST (glutathione S-transferase) pull-down assay using HEK-293T (human embryonic kidney 293T) cell lysates, endogenous TSG101 and two other exogenously expressed ESCRT-I components [hVps28 (human vacuolar protein sorting 28) and hVps37A] were shown to associate with GST-ALG-2 in the presence of Ca2+. By the yeast two-hybrid assay, however, a positive interaction was observed with only TSG101 among the three ESCRT-I components, suggesting that ALG-2 associates with hVps28 and hVps37A indirectly through TSG101. Using various deletion mutants of TSG101, the central PRR (proline-rich region) was found to be sufficient for interaction with ALG-2 by the GST-pull-down assay. Direct binding of ALG-2 to the TSG101 PRR was demonstrated by an overlay assay using biotin-labelled ALG-2 as a probe. In immunofluorescence microscopic analysis of HeLa cells that overexpressed a GFP (green fluorescent protein)-fused ATPase-defective dominant-negative form of SKD1/Vps4B (GFP-SKD1(E235Q)), ALG-2 exhibited a punctate distribution at the perinuclear area and co-localized with GFP-SKD1(E235Q) to aberrant endosomes. This punctate distribution of ALG-2 was markedly diminished by treatment of HeLa cells with a membrane-permeant Ca2+ chelator. Moreover, a Ca2+-binding-defective mutant of ALG-2 did not co-localize with GFP-SKD1(E235Q). Our findings suggest that ALG-2 may function as a Ca2+-dependent accessory protein of the endosomal sorting machinery by interacting directly with TSG101 as well as with Alix.

Increased Alix (apoptosis-linked gene-2 interacting protein X) immunoreactivity in the degenerating striatum of rats chronically treated by 3-nitropropionic acid

Chronic intoxication by 3-nitropropionic acid in the Lewis rat reproduces many features reminiscent of Huntington's disease including behavioural alterations and cortico-striatal degeneration. In particular, in this model, striatal degeneration is accompanied by calpain activation as found in the human disease. The present study was undertaken to determine whether the expression of Alix (apoptosis linked gene-2 interacting protein), a widespread protein involved in neuronal death, would be modified in the striatum and cortex of 3NP-treated rats. The results clearly show that Alix immunoreactivity is increased in the neuronal cell bodies of the lateral striatum, where degeneration is massive. The medial striatum and the cortex that lack neurodegeneration remain only weakly labelled. This is further evidence suggesting an involvement of Alix in the events driving neuronal death.

Ca2+ binding to EF hands 1 and 3 is essential for the interaction of apoptosis-linked gene-2 with Alix/AIP1 in ocular melanoma

Apoptosis-linked gene-2 (ALG-2) encodes a 22 kDa Ca(2+)-binding protein of the penta EF-hand family that is required for programmed cell death in response to various apoptotic agents. Here, we demonstrate that ALG-2 mRNA and protein are down-regulated in human uveal melanoma cells compared to their progenitor cells, normal melanocytes. The down regulation of ALG-2 may provide melanoma cells with a selective advantage. ALG-2 and its putative target molecule, Alix/AIP1, are localized primarily in the cytoplasm of melanocytes and melanoma cells independent of the intracellular Ca(2+) concentration or the activation of apoptosis. Cross-linking and analytical centrifugation studies support a single-species dimer conformation of ALG-2, also independent of Ca(2+) concentration. However, binding of Ca(2+) to both EF-1 and EF-3 is necessary for ALG-2 interaction with Alix/AIP1 as demonstrated using surface plasmon resonance spectroscopy. Mutations in EF-5 result in reduced target interaction without alteration in Ca(2+) affinity. The addition of N-terminal ALG-2 peptides, residues 1-22 or residues 7-17, does not alter the interaction of ALG-2 or an N-terminal deletion mutant of ALG-2 with Alix/AIP1, as might be expected from a model derived from the crystal structure of ALG-2. Fluorescence studies of ALG-2 demonstrate that an increase in surface hydrophobicity is primarily due to Ca(2+) binding to EF-3, while Ca(2+) binding to EF-1 has little effect on surface exposure of hydrophobic residues. Together, these data indicate that gross surface hydrophobicity changes are insufficient for target recognition.

Molecular and cellular profiles of insect bacteriocytes: mutualism and harm at the initial evolutionary step of symbiogenesis

Intracellular symbiosis is considered to be a driving force in eukaryotic cell evolution. In insects, little is known about the molecular bases of the bacteria-bearing host cells (bacteriocytes), particularly in the initial steps of symbiosis, where the bacterial genome has not experienced severe gene deletions because of evolutionary constraints associated with intracellular and vertical transmission. Here, we have applied polymerase chain reaction (PCR)-subtracted cDNA and reverse Northern analysis on the bacteriocytes of a recently established endosymbiosis, the weevil Sitophilus zeamais, to discover genes of potential relevance to bacteriocyte genetics. We provide a broad characterization of bacteriocyte transcriptional responses to intracellular bacteria, including pathways covering metabolism-transport-stress (MTS), cell signalling and trafficking, growth and apoptosis, as well as innate immunity. MTS genes show an intriguing diabetes-like pathogenic profile associated with increased stress, as indicated by high levels of upregulations of carbohydrate transporters, aldose reductases and stress-related genes. A high-performance liquid chromatography (HPLC) analysis of tissue carbohydrate contents highlighted an increased carbohydrate assimilation in symbiotic insects and the prevalence of a polyol biosynthetic pathway, as indicated by the accumulation of sorbitol, mannitol and fructose in the bacteriocytes. These findings provide the first genetic perspectives on the nature of the interaction between insect and cooperative bacteria. They unravel the profound insect bacteriocyte stress associated with increased metabolism and cell trafficking, and they shed light on the potential role of the innate immunity during the pathogeny-mutualism transition at the initial stage of insect symbiogenesis.

Evaluation of gene expression in pigs selected for enhanced reproduction using differential display PCR and human microarrays: I. Ovarian follicles1,2

Differential display PCR (ddPCR) and complementary DNA microarray analyses were used to evaluate gene expression differences in porcine ovarian follicles between a line of pigs selected for an index of ovulation rate and embryo survival (Line I) and its randomly selected control line (Line C). Follicles (4.0 to 7.0 mm) were dissected from ovaries of multiparous sows (n = 27) at either 2 or 4 d following PGF2alpha analog injection on d 12 to 14 of the estrous cycle. Using ddPCR, differentially expressed bands (n = 282) were excised from gels and 107 were sequenced, yielding 84 unique porcine follicle expressed sequence tags. Northern hybridization confirmed differential expression (between lines, days, or follicle sizes) for messenger RNA representing the calpain I light subunit, cytochrome C oxidase subunit III, cytochrome P450 aromatase, and cytochrome P450 side chain cleavage genes. For microarray analysis, two mRNA pools representing follicles (d 2; 4.50 to 4.75 mm) from Line I and Line C sows were hybridized to the Incyte UniGEM V1.0 human chip (approximately 7,000 gene probes). A second analysis was performed using mRNA from follicles (d 2; 4.50 to 5.00 mm) hybridized to the Incyte UniGEM V2.0 human chip (approximately 9,100 gene probes). A total of 33 and 21 genes were identified with significant expression differences using UniGEM V1.0 and V2.0, respectively (twofold or greater relative expression following adjustment for expression of control probes). However, there was little overlap between results of the two hybridizations. Expression differences between lines for two genes, follistatin and nuclear receptor subfamily 4, group A, member 1, were confirmed using Northern hybridization. These results demonstrate changes in follicular gene expression as the result of long-term selection for enhanced reproduction. These correlated responses may directly represent allelic variation utilized by selection (e.g., quantitative trait loci), or more likely, transcriptional changes in other genes that interact with reproductive QTL. This work represents one of the first applications of gene expression analysis to evaluate long-term selection response in livestock populations.

Information transfer in the penta-EF-hand protein sorcin does not operate via the canonical structural/functional pairing. A study with site-specific mutants

Sorcin is a typical penta-EF-hand protein that participates in Ca2+-regulated processes by translocating reversibly from cytosol to membranes, where it interacts with different target proteins in different tissues. Binding of two Ca2+/monomer triggers translocation, although EF1, EF2, and EF3 are potentially able to bind calcium at micromolar concentrations. To identify the functional pair, the conserved bidentate -Z glutamate in these EF-hands was mutated to yield E53Q-, E94A-, and E124A-sorcin, respectively. Limited structural perturbations occur only in E124A-sorcin due to involvement of Glu-124 in a network of interactions that comprise the long D helix connecting EF3 to EF2. The overall affinity for Ca2+ and for two sorcin targets, annexin VII and the ryanodine receptor, follows the order wild-type > E53Q- > E94A- > E124A-sorcin, indicating that disruption of EF3 has the largest functional impact and that disruption of EF2 and EF1 has progressively smaller effects. Based on this experimental evidence, EF3 and EF2, which are not paired in the canonical manner, are the functional EF-hands. Sorcin is proposed to be activated upon Ca2+ binding to EF3 and transmission of the conformational change at Glu-124 via the D helix to EF2 and from there to EF1 via the canonical structural/functional pairing. This mechanism may be applicable to all penta-EF-hand proteins.

pH regulation in Aspergillus and parallels with higher eukaryotic regulatory systems

All organisms respond to their environment to some extent, and for many microbes the variation in environment can be enormous. An important asset for coping with environmental variation is physiological versatility--a hallmark of many fungi. The ability of fungi to thrive over a wide range of pH is partly due to a genetic regulatory system that tailors gene expression to the ambient pH. Here we focus on the pH regulatory system of Aspergillus nidulans, where a novel signal transduction (pal) pathway mediates the first of two steps in the proteolytic processing of a transcription factor (PacC). Such processing is reminiscent of that of some well-known higher eukaryotic transcription factors, such as Cubitus interruptus, NF-kappa B and sterol regulatory element binding proteins. Intriguingly, endocytosis seems to be connected to pH signalling.

YPXL/I is a protein interaction motif recognized by aspergillus PalA and its human homologue, AIP1/Alix

The zinc finger transcription factor PacC undergoes two-step proteolytic activation in response to alkaline ambient pH. PalA is a component of the fungal ambient pH signal transduction pathway. Its mammalian homologue AIP1/Alix interacts with the apoptosis-linked protein ALG-2. We show that both PalA and AIP1/Alix recognize a protein-protein binding motif that we denote YPXL/I, where Tyr, Pro, and Leu/Ile are crucial for its interactive properties. Two such motifs flanking the signaling protease cleavage site mediate direct binding of PalA to PacC, required for the first and only pH-regulated cleavage of this transcription factor. PalA can bind the "closed" (i.e., wild-type full-length) conformer of PacC, suggesting that PalA binding constitutes the first stage in the two-step proteolytic cascade, recruiting or facilitating access of the signaling protease, presumably PalB. In addition to recognizing YPXL/I motifs, both PalA and AIP1/Alix interact with the Aspergillus class E Vps protein Vps32 homologue, a member of a protein complex involved in the early steps of the multivesicular body pathway, suggesting that this interaction is an additional feature of proteins of the PalA/AIP1/Alix family.

Structures, functions and molecular evolution of the penta-EF-hand Ca2+-binding proteins

Penta-EF-hand (PEF) proteins comprise a family of Ca(2+)-binding proteins that have five repetitive EF-hand motifs. Among the eight alpha-helices (alpha1-alpha8), alpha4 and alpha7 link EF2-EF3 and EF4-EF5, respectively. In addition to the structural similarities in the EF-hand regions, the PEF protein family members have common features: (i) dimerization through unpaired C-terminal EF5s, (ii) possession of hydrophobic Gly/Pro-rich N-terminal domains, and (iii) Ca(2+)-dependent translocation to membranes. Based on comparison of amino acid sequences, mammalian PEF proteins are classified into two groups: Group I PEF proteins (ALG-2 and peflin) and Group II PEF proteins (Ca(2+)-dependent protease calpain subfamily members, sorcin and grancalcin). The Group I genes have also been found in lower animals, plants, fungi and protists. Recent findings of specific interacting proteins have started to gradually unveil the functions of the noncatalytic mammalian PEF proteins.

ALG-2: a Ca2+ -binding modulator protein involved in cell proliferation and in cell death

During the development of an organism, cell proliferation, differentiation and cell death are tightly balanced, and are controlled by a number of different regulators. Alterations in this balance are often observed in a variety of human diseases. The role of Ca(2+) as one of the key regulators of the cell is discussed with respect to a recently discovered Ca(2+)-binding protein, ALG-2, which is highly upregulated in cancerous tissues of different origins. The role of ALG-2 as a possible clinical marker and, molecularly, as a possible modulator at the interface between cell proliferation and cell death is discussed.

Preparation of monoclonal antibody against apoptosis-associated antigens of hepatoma cells by subtractive immunization

AIM: To elucidate the expression of the apoptosis-associated molecules in human primary hepatocellular carcinoma (HCC) cells, and prepare the monoclonal antibodies (mAb) against the apoptosis-associated antigens of HCC cells.

METHODS: Human HCC cell line HCC-9204 cells were induced apoptosis with 60 mL•L^-1 ethanol for 6 h and their morphological changes were observed by transmission electron microscope. The cell DNA fragmentations were detected by Terminal Deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay, and the cell DNA contents by flow cytometry. Ten mice were immunized with ethanol-induced apoptotic HCC-9204 cells with the method of subtractive immunization, while the other 10 mice used as the control were immunized by the routine procedures. The tail blood of all the mice were prepared after the last immunization, and the produced antibodies were determined by the immunocytochemical ABC staining. The splenic cells of the mice whose tail blood sera-HCC-9204 cells serum reactions were most different between the apoptotic and the non-apoptotic were prepared and fused with the mouse myeloma cell line SP2/0 cells. The positive antibodies were selected by ELISA assay. The fusion rates of hybridoma cells and the producing rates of antibodies were calculated. The fused cells that secreted candidate objective antibody were cloned continually with the of limited dilution method, and then selected and analyzed further by the immunocytochemical ABC staining. The chromosomes of the cloned hybridoma cells that secreted objective mAb and the mAb immunoglobulin (Ig) subtype of the prepared mAb were also determined. The molecular mass of the mAb associated antigen was analyzed by Western blot assay.

RESULTS: HCC-9204 cells treated with 60 mL•L^-1 ethanol for 6 h, manifested obvious apoptotic morphological changes, the majority of the cells were TUNEL-positive, and the sub-G1 apoptotic peak was evident. There were 2 mice in the experimental group whose tail blood serum reacted strongly with the apoptotic HCC-9204 cells, but weakly with their non-apoptotic counterparts. In the fusion rates of hybridoma cells as well as the producing rates of the antibody deseribed above, there did not show significant difference between the experimental and the control group, but weakly with non-apoptotic HCC-9204. However, the total producing rate of antibodies in the experimental group was significantly lower compared with the control (P < 0.01), and so was the producing rate of the antibodies which reacted strongly with both apoptotic and non-apoptotic HCC-9204 cells (P < 0.01). After cloned continually for several times the cell that produce mAb which reacted strongly with the nuclei of ethanol-induced apoptotic HCC-9204 cells, but very weakly with that of non-apoptotic cells was selected out. Chromosome analysis revealed that the selected cell was with the universal characteristics of the monoclonal hybridoma cells which secreted mAb, and the Ig subtype of the prepared mAb was IgG1. The molecular mass of this mAb associated antigen of was about 75 ku.

CONCLUSION: Subtractive immunization is a useful method to prepare the mAb against the apoptosis-associated antigens of cells. The expression of some molecules increases to some extent in HCC-9204 cells in the process of apoptosis induced by low-concentration ethanol. The mAb that may be against ethanol-induced apoptosis-associated antigens of HCC cells was successfully prepared and primarily identified.

Interaction of ALG-2 with ASK1 influences ASK1 localization and subsequent JNK activation

ALG-2 (apoptosis linked gene-2) is an essential protein for the execution of apoptosis whose function is largely unknown. Here, we demonstrate that ALG-2 could interact with the C-terminus (amino acids 941-1375) of the apoptosis signal-regulating kinase 1 (ASK1) in BOSC23 cells as well as in vitro. ASK1 failed to bind to an isotype of ALG-2 found in the liver, ALG-2,1, in which two amino acids (Gly-121 and Phe-122) are deleted. This implies that the interaction is very specific. Cotransfection with ALG-2 resulted in the nuclear presence of ASK1 and inhibited the activation of c-Jun N-terminal kinase (JNK) by ASK1 in BOSC23 cells. This study reports that ALG-2 could regulate the subcellular localization and the JNK activity modulation of ASK1 by direct interaction.