Molecular cloning and characterization of a cellular protein that interacts with the human immunodeficiency virus type 1 Tat transactivator and encodes a strong transcriptional activation domain

The Interactions of the Complement System with Human Cytomegalovirus

The complement system is an evolutionarily ancient component of innate immunity that serves as an important first line of defense against pathogens, including viruses. In response to infection, the complement system can be activated by three distinct yet converging pathways (classical, lectin, and alternative) capable of engaging multiple antiviral host responses to confront acute, chronic, and recurrent viral infections. Complement can exert profound antiviral effects via multiple mechanisms including the induction of inflammation and chemotaxis to sites of infection, neutralization/opsonization of viruses and virally infected cells, and it can even shape adaptive immune responses. With millions of years of co-evolution and the ability to establish life-long infections, herpesviruses have evolved unique mechanisms to counter complement-mediated antiviral defenses, thus enabling their survival and replication within humans. This review aims to comprehensively summarize how human herpesviruses engage with the complement system and highlight our understanding of the role of complement in human cytomegalovirus (HCMV) infection, immunity, and viral replication. Herein we describe the novel and unorthodox roles of complement proteins beyond their roles in innate immunity and discuss gaps in knowledge and future directions of complement and HCMV research.

The chaperone protein p32 stabilizes HIV-1 Tat and strengthens the p-TEFb/RNAPII/TAR complex promoting HIV transcription elongation

HIV gene expression is modulated by the combinatorial activity of the HIV transcriptional activator, Tat, host transcription factors, and chromatin remodeling complexes. To identify host factors regulating HIV transcription, we used specific single-guide RNAs and endonuclease-deficient Cas9 to perform chromatin affinity purification of the integrated HIV promoter followed by mass spectrometry. The scaffold protein, p32, also called ASF/SF2 splicing factor-associated protein, was identified among the top enriched factors present in actively transcribing HIV promoters but absent in silenced ones. Chromatin immunoprecipitation analysis confirmed the presence of p32 on active HIV promoters and its enhanced recruitment by Tat. HIV uses Tat to efficiently recruit positive transcription elongation factor b (p-TEFb) (CDK9/CCNT1) to TAR, an RNA secondary structure that forms from the first 59 bp of HIV transcripts, to enhance RNAPII transcriptional elongation. The RNA interference of p32 significantly reduced HIV transcription in primary CD4+T cells and in HIV chronically infected cells, independently of either HIV splicing or p32 anti-splicing activity. Conversely, overexpression of p32 specifically increased Tat-dependent HIV transcription. p32 was found to directly interact with Tat's basic domain enhancing Tat stability and half-life. Conversely, p32 associates with Tat via N- and C-terminal domains. Likely due its scaffold properties, p32 also promoted Tat association with TAR, p-TEFb, and RNAPII enhancing Tat-dependent HIV transcription. In sum, we identified p32 as a host factor that interacts with and stabilizes Tat protein, promotes Tat-dependent transcriptional regulation, and may be explored for HIV-targeted transcriptional inhibition.

Affinity Purification-Mass Spectrometry Identifies a Novel Interaction between a Polerovirus and a Conserved Innate Immunity Aphid Protein that Regulates Transmission Efficiency

The vast majority of plant viruses are transmitted by insect vectors, with many crucial aspects of the transmission process being mediated by key protein-protein interactions. Still, very few vector proteins interacting with viruses have been identified and functionally characterized. Potato leafroll virus (PLRV) is transmitted most efficiently by Myzus persicae, the green peach aphid, in a circulative, non-propagative manner. Using affinity purification coupled to high-resolution mass spectrometry (AP-MS), we identified 11 proteins from M. persicaedisplaying a high probability of interaction with PLRV and an additional 23 vector proteins with medium confidence interaction scores. Three of these aphid proteins were confirmed to directly interact with the structural proteins of PLRV and other luteovirid species via yeast two-hybrid. Immunolocalization of one of these direct PLRV-interacting proteins, an orthologue of the human innate immunity protein complement component 1 Q subcomponent-binding protein (C1QBP), shows that MpC1QBP partially co-localizes with PLRV in cytoplasmic puncta and along the periphery of aphid gut epithelial cells. Artificial diet delivery to aphids of a chemical inhibitor of C1QBP leads to increased PLRV acquisition by aphids and subsequently increased titer in inoculated plants, supporting a role for C1QBP in the acquisition and transmission efficiency of PLRV by M. persicae. This study presents the first use of AP-MS for the in vivo isolation of a functionally relevant insect vector-virus protein complex. MS data are available from ProteomeXchange.org using the project identifier PXD022167.

Arginase II protein regulates Parkin-dependent p32 degradation that contributes to Ca2+-dependent eNOS activation in endothelial cells

Aims

Arginase II (ArgII) plays a key role in the regulation of Ca²⁺ between the cytosol and mitochondria in a p32-dependent manner. p32 contributes to endothelial nitric oxide synthase (eNOS) activation through the Ca²⁺/CaMKII/AMPK/p38MAPK/Akt signalling cascade. Therefore, we investigated a novel function of ArgII in the regulation of p32 stability.

Methods and results

mRNA levels were measured by quantitative reverse transcription-PCR, and protein levels and activation were confirmed by western blot analysis. Ca²⁺ concentrations were measured by FACS analysis and a vascular tension assay was performed. ArgII bound to p32, and ArgII protein knockdown using siArgII facilitated the ubiquitin-dependent proteasomal degradation of p32. β-lactone, a proteasome inhibitor, inhibited the p32 degradation associated with endothelial dysfunction in a Ca²⁺-dependent manner. The amino acids Lys154, Lys 180, and Lys220 of the p32 protein were identified as putative ubiquitination sites. When these sites were mutated, p32 was resistant to degradation in the presence of siArgII, and endothelial function was impaired. Knockdown of Pink/Parkin as an E3-ubiquitin ligase with siRNAs resulted in increased p32, decreased [Ca²⁺]c, and attenuated CaMKII-dependent eNOS activation by siArgII. siArgII-dependent Parkin activation was attenuated by KN93, a CaMKII inhibitor. Knockdown of ArgII mRNA and its gene, but not inhibition of its activity, accelerated the interaction between p32 and Parkin and reduced p32 levels. In aortas of ArgII^−/− mice, p32 levels were reduced by activated Parkin and inhibition of CaMKII attenuated Parkin-dependent p32 lysis. siParkin blunted the phosphorylation of the activated CaMKII/AMPK/p38MAPK/Akt/eNOS signalling cascade. However, ApoE^−/− mice fed a high-cholesterol diet had greater ArgII activity, significantly attenuated phosphorylation of Parkin, and increased p32 levels. Incubation with siArgII augmented p32 ubiquitination through Parkin activation, and induced signalling cascade activation.

Conclusion

The results suggest a novel function for ArgII protein in Parkin-dependent ubiquitination of p32 that is associated with Ca²⁺-mediated eNOS activation in endothelial cells.

Overexpressed p32 localized in the endoplasmic reticulum and mitochondria negatively regulates calcium‑dependent endothelial nitric oxide synthase activit

The p32 protein plays a crucial role in the regulation of cytosolic Ca²⁺ concentrations ([Ca2+]c) that contributes to the Ca²⁺-dependent signaling cascade. Using an adenovirus and plasmid p32-overexpression system, the aim of the study was to evaluate the role of p32 in the regulation of [Ca²⁺] and its potential associated with Ca²⁺-dependent endothelial nitric oxide synthase (eNOS) activation in endothelial cells. Using electron and confocal microscopic analysis, p32 overexpression was observed to be localized to mitochondria and the endoplasmic reticulum and played an important role in Ca²⁺ translocation, resulting in increased [Ca²⁺] in these organelles and reducing cytosolic [Ca²⁺] ([Ca²⁺]c). This decreased [Ca²⁺]c following p32 overexpression attenuated the Ca²⁺-dependent signaling cascade of calcium/calmodulin dependent protein kinase II (CaMKII)/AKT/eNOS phosphorylation. Moreover, in aortic endothelia of wild-type mice intravenously administered adenovirus encoding the p32 gene, increased p32 levels reduced NO production and accelerated reactive oxygen species (ROS) generation. In a vascular tension assay, p32 overexpression decreased acetylcholine (Ach)-induced vasorelaxation and augmented phenylephrine (PE)-dependent vasoconstriction. Notably, decreased levels of arginase II (ArgII) protein using siArgII were associated with downregulation of overexpressed p32 protein, which contributed to CaMKII-dependent eNOS phosphorylation at Ser1177. These results indicated that increased protein levels of p32 caused endothelial dysfunction through attenuation of the Ca²⁺-dependent signaling cascade and that ArgII protein participated in the stability of p32. Therefore, p32 may be a novel target for the treatment of vascular diseases associated with endothelial disorders.

Identification of the hyaluronic acid pathway as a therapeutic target for facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is linked to epigenetic derepression of the germline/embryonic transcription factor DUX4 in skeletal muscle. However, the etiology of muscle pathology is not fully understood, as DUX4 misexpression is not tightly correlated with disease severity. Using a DUX4-inducible cell model, we show that multiple DUX4-induced molecular pathologies that have been observed in patient-derived disease models are mediated by the signaling molecule hyaluronic acid (HA), which accumulates following DUX4 induction. These pathologies include formation of RNA granules, FUS aggregation, DNA damage, caspase activation, and cell death. We also observe previously unidentified pathologies including mislocalization of mitochondria and the DUX4- and HA-binding protein C1QBP. These pathologies are prevented by 4-methylumbelliferone, an inhibitor of HA biosynthesis. Critically, 4-methylumbelliferone does not disrupt DUX4-C1QBP binding and has only a limited effect on DUX4 transcriptional activity, establishing that HA signaling has a central function in pathology and is a target for FSHD therapeutics.

p32 is a negative regulator of p53 tetramerization and transactivation

p53 is a sequence-specific transcription factor, and proper regulation of p53 transcriptional activity is critical for orchestrating different tumor-suppressive mechanisms. p32 is a multifunctional protein which interacts with a large number of viral proteins and transcription factors. Here, we investigate the effect of p32 on p53 transactivation and identify a novel mechanism by which p32 alters the functional characteristics of p53. Specifically, p32 attenuates p53-dependent transcription through impairment of p53 binding to its response elements on target genes. Upon p32 expression, p53 levels bound at target genes are decreased, and p53 target genes are inactivated, strongly indicating that p32 restricts p53 occupancy and function at target genes. The primary mechanism contributing to the observed action of p32 is the ability of p32 to interact with the p53 tetramerization domain and to block p53 tetramerization, which in turn enhances nuclear export and degradation of p53, leading to defective p53 transactivation. Collectively, these data establish p32 as a negative regulator of p53 function and suggest the therapeutic potential of targeting p32 for cancer treatment.

Regulation of HIV-1 transcription

Human immunodeficiency virus type-1 (HIV-1) is a highly pathogenic lentivirus that requires transcription of its provirus genome for completion of the viral life cycle and the production of progeny virions. Since the first genetic analysis of HIV-1 in 1985, much has been learned about the transcriptional regulation of the HIV-1 genome in infected cells. It has been demonstrated that HIV-1 transcription depends on a varied and complex interaction of host cell transcription factors with the viral long terminal repeat (LTR) promoter. The regulatory elements within the LTR interact with constitutive and inducible transcription factors to direct the assembly of a stable transcription complex that stimulates multiple rounds of transcription by RNA polymerase II (RNAPII). However, the majority of these transcripts terminate prematurely in the absence of the virally encoded trans-activator protein Tat, which stimulates HIV-1 transcription elongation by interacting with a stem-loop RNA element (TAR) formed at the extreme 5' end of all viral transcripts. The Tat-TAR interaction recruits a cellular kinase into the initiation-elongation complex that alters the elongation properties of RNAPII during its transit through TAR. This review summarizes our current knowledge and understanding of the regulation of HIV-1 transcription in infected cells and highlights the important contributions human lentivirus gene regulation has made to our general understanding of the transcription process.

Sequential steps in Tat trans-activation of HIV-1 mediated through cellular DNA, RNA, and protein binding factors

The regulation of HIV expression is controlled by the activity of the Long Terminal Repeat (LTR). Trans-activation by the virally encoded Tat protein is one of the main mechanisms of LTR activation. Tat binds to its target, TAR RNA, and cellular proteins that bind the LTR, Tat, or TAR RNA are important components of the trans-activation process. We will review the factors that have been characterized for a possible involvement in this mechanism. Whereas LTR binding proteins consist of Sp1 and TBP, a large number of factors that bind TAR RNA have been isolated. We have previously cloned two of them by RNA probe recognition: TRBP and La. We have shown that the in vitro and in vivo binding of TRBP to TAR RNA correlates with a constant expression of the protein during HIV-1 infection. Several proteins that interact with Tat have mainly positive, but some negative, effects on trans-activation. Genetic studies have defined that human chromosome 12 encodes a protein that will allow trans-activation in rodent cells. The binding and the functional data about these proteins suggest sequential steps for the Tat trans-activation mechanism. Each of these intracellular molecular events could be the target for molecular intervention against the virus.

Mam33 promotes cytochrome c oxidase subunit I translation in Saccharomyces cerevisiae mitochondria

Expression of genes encoded by the mitochondrial genome is dependent on gene-specific translational activators. Mam33, the yeast homologue of p32/gC1qR/C1QBP/HABP1, promotes the translation of Cox1, a core catalytic subunit of respiratory chain complex IV.

Three mitochondrial DNA–encoded proteins, Cox1, Cox2, and Cox3, comprise the core of the cytochrome c oxidase complex. Gene-specific translational activators ensure that these respiratory chain subunits are synthesized at the correct location and in stoichiometric ratios to prevent unassembled protein products from generating free oxygen radicals. In the yeast Saccharomyces cerevisiae, the nuclear-encoded proteins Mss51 and Pet309 specifically activate mitochondrial translation of the largest subunit, Cox1. Here we report that Mam33 is a third COX1 translational activator in yeast mitochondria. Mam33 is required for cells to adapt efficiently from fermentation to respiration. In the absence of Mam33, Cox1 translation is impaired, and cells poorly adapt to respiratory conditions because they lack basal fermentative levels of Cox1.

p32 is a novel target for viral protein ICP34.5 of herpes simplex virus type 1 and facilitates viral nuclear egress

As a large double-stranded DNA virus, herpes simplex virus type 1 (HSV-1) assembles capsids in the nucleus where the viral particles exit by budding through the inner nuclear membrane. Although a number of viral and host proteins are involved, the machinery of viral egress is not well understood. In a search for host interacting proteins of ICP34.5, which is a virulence factor of HSV-1, we identified a cellular protein, p32 (gC1qR/HABP1), by mass spectrophotometer analysis. When expressed, ICP34.5 associated with p32 in mammalian cells. Upon HSV-1 infection, p32 was recruited to the inner nuclear membrane by ICP34.5, which paralleled the phosphorylation and rearrangement of nuclear lamina. Knockdown of p32 in HSV-1-infected cells significantly reduced the production of cell-free viruses, suggesting that p32 is a mediator of HSV-1 nuclear egress. These observations suggest that the interaction between HSV-1 ICP34.5 and p32 leads to the disintegration of nuclear lamina and facilitates the nuclear egress of HSV-1 particles.

Drosophila TAP/p32 is a core histone chaperone that cooperates with NAP-1, NLP, and nucleophosmin in sperm chromatin remodeling during fertilization

Nuclear DNA in the male gamete of sexually reproducing animals is organized as sperm chromatin compacted primarily by sperm-specific protamines. Fertilization leads to sperm chromatin remodeling, during which protamines are expelled and replaced by histones. Despite our increased understanding of the factors that mediate nucleosome assembly in the nascent male pronucleus, the machinery for protamine removal remains largely unknown. Here we identify four Drosophila protamine chaperones that mediate the dissociation of protamine-DNA complexes: NAP-1, NLP, and nucleophosmin are previously characterized histone chaperones, and TAP/p32 has no known function in chromatin metabolism. We show that TAP/p32 is required for the removal of Drosophila protamine B in vitro, whereas NAP-1, NLP, and Nph share roles in the removal of protamine A. Embryos from P32-null females show defective formation of the male pronucleus in vivo. TAP/p32, similar to NAP-1, NLP, and Nph, facilitates nucleosome assembly in vitro and is therefore a histone chaperone. Furthermore, mutants of P32, Nlp, and Nph exhibit synthetic-lethal genetic interactions. In summary, we identified factors mediating protamine removal from DNA and reconstituted in a defined system the process of sperm chromatin remodeling that exchanges protamines for histones to form the nucleosome-based chromatin characteristic of somatic cells.

A role for the mitochondrial-associated protein p32 in regulation of trophoblast proliferation

p32 is a conserved eukaryotic protein which is primarily expressed in the mitochondria and regulates cell proliferation, migration and metabolism in various tissues. In this study, we sought to examine the expression and function of p32 in the human placenta. p32 was highly expressed in the syncytiotrophoblast, the underlying cytotrophoblast (CTB), the vascular endothelium and by a proportion of cells in the villous stroma in first trimester and term placenta. p32 mRNA and protein expression was significantly higher in the first trimester of pregnancy than at term, and expression in the trophoblast was significantly reduced in placentas from women with fetal growth restriction (FGR). Small interfering RNA (siRNA)-mediated knockdown of p32 in term placental explants significantly reduced the number of Ki67-positive CTB, but did not alter CTB apoptosis or necrosis. p32 knockdown increased lactate production, reduced glucose extraction from culture medium and was associated with reduced MitoTracker dye accumulation in trophoblast mitochondria. p32 knockdown was also associated with a significant reduction in expression of the mitochondrial respiratory complexes I and IV. These data suggest that p32 expression is important for CTB proliferation, via a mechanism involving regulation of normal mitochondrial function. As p32 expression is reduced in FGR placentas, this may contribute to some of the observed placental pathology, such as reduced CTB proliferation and mitochondrial dysfunction.

Interaction network of proteins associated with human cytomegalovirus IE2-p86 protein during infection: a proteomic analysis

Human cytomegalovirus protein IE2-p86 exerts its functions through interaction with other viral and cellular proteins. To further delineate its protein interaction network, we generated a recombinant virus expressing SG-tagged IE2-p86 and used tandem affinity purification coupled with mass spectrometry. A total of 9 viral proteins and 75 cellular proteins were found to associate with IE2-p86 protein during the first 48 hours of infection. The protein profile at 8, 24, and 48 h post infection revealed that UL84 tightly associated with IE2-p86, and more viral and cellular proteins came into association with IE2-p86 with the progression of virus infection. A computational analysis of the protein-protein interaction network indicated that all of the 9 viral proteins and most of the cellular proteins identified in the study are interconnected to varying degrees. Of the cellular proteins that were confirmed to associate with IE2-p86 by immunoprecipitation, C1QBP was further shown to be upregulated by HCMV infection and colocalized with IE2-p86, UL84 and UL44 in the virus replication compartment of the nucleus. The IE2-p86 interactome network demonstrated the temporal development of stable and abundant protein complexes that associate with IE2-p86 and provided a framework to benefit future studies of various protein complexes during HCMV infection.

Characterization of complement 1q binding protein of tiger shrimp, Penaeus monodon, and its C1q binding activity

The receptor for the globular heads of C1q, C1qBP/gC1qR/p33, is a multicompartmental, multifunctional cellular protein with an important role in infection and in inflammation. In the present study, we identified and characterized the complement component 1q subcomponent binding protein (C1qBP) from the tiger shrimp Penaeus monodon (designated as PmC1qBP). The open reading frame of PmC1qBP encodes 262 amino acid residues with a conserved MAM33 domain, an arginine-glycine-aspartate cell adhesion motif, and a mitochondrial targeting sequence in the first 53 amino acids. PmC1qBP shares 32%-81% similarity with known C1qBPs and clusters with lobster gC1qR under phylogenetic analysis. The temporal PmC1qBP mRNA expression in the hepatopancreas was significantly enhanced at 9 h after Vibrio vulnificus challenge. The native PmC1qBP was expressed in the gills, hepatopancreas, ovaries, and intestines as a precursor (38 kDa) and the active peptide (35 kDa). The recombinant PmC1qBP protein was expressed in Escherichia coli BL21, and was purified using nickel-nitrilotriacetic acid agarose. A complement 1q binding assay indicated that the rC1qBP protein competitively binds to C1q in mouse serum. The data reveal that PmC1qBP is not only involved in shrimp immune responses to pathogenic infections, but also cross-binding to the mouse C1q.

Identification of human host proteins contributing to H5N1 influenza virus propagation by membrane proteomics

The highly pathogenic avian influenza (HPAI) H5N1 virus is a highly virulent pathogen that causes respiratory diseases and death in humans and other animal species worldwide. Because influenza is an enveloped virus, the entry, assembly, and budding of virus particles are essential steps in the viral life cycle, and the virus relies on the participation of host cellular membrane proteins for all of these steps. Thus, we took a comparative membrane proteomics approach by using 2-DE coupled with MALDI-TOF/TOF MS to profile membrane proteins involved in H5N1 virus infection at 6, 12, and 24 h. Forty-two different proteins were found to vary on A549 cells due to H5N1 virus infection. Of these proteins, 57% were membrane or membrane-associated proteins. To further characterize the roles of novel identified proteins in virus propagation, the siRNA technology were applied and complement component C1q binding protein, annexin 2, prohibitin, peroxiredoxin 1 and heat shock protein 90-beta were successfully demonstrated to be contributed to viral propagation. In conclusion, the present study provides important new insight into understanding the roles of host membrane proteins in viral infection progress, and this insight is of particular importance for the development of novel therapeutic strategies.

Binding of cellular p32 protein to the rubella virus P150 replicase protein via PxxPxR motifs

A proline-rich region (PRR) within the rubella virus (RUBV) P150 replicase protein that contains three SH3 domain-binding motifs (PxxPxR) was investigated for its ability to bind cell proteins. Pull-down experiments using a glutathione S-transferase-PRR fusion revealed PxxPxR motif-specific binding with human p32 protein (gC1qR), which could be mediated by either of the first two motifs. This finding was of interest because p32 protein also binds to the RUBV capsid protein. Binding of p32 to P150 was confirmed and was abolished by mutation of the first two motifs. When mutations in the first two motifs were introduced into a RUBV cDNA infectious clone, virus replication was significantly impaired. However, virus RNA synthesis was found to be unaffected, and subsequent immunofluorescence analysis of RUBV-infected cells revealed co-localization of p32 and P150 but little overlap of p32 with RNA replication complexes, indicating that p32 does not participate directly in virus RNA synthesis. Thus, the role of p32 in RUBV replication remains unresolved.

Genetic analysis in Drosophila reveals a role for the mitochondrial protein p32 in synaptic transmission

Mitochondria located within neuronal presynaptic terminals have been shown to play important roles in the release of chemical neurotransmitters. In the present study, a genetic screen for synaptic transmission mutants of Drosophila has identified the first mutation in a Drosophila homolog of the mitochondrial protein P32. Although P32 is highly conserved and has been studied extensively, its physiological role in mitochondria remains unknown and it has not previously been implicated in neural function. The Drosophila P32 mutant, referred to as dp32^EC1, exhibited a temperature-sensitive (TS) paralytic behavioral phenotype. Moreover, electrophysiological analysis at adult neuromuscular synapses revealed a TS reduction in the amplitude of excitatory postsynaptic currents (EPSC) and indicated that dP32 functions in neurotransmitter release. These studies are the first to address P32 function in Drosophila and expand our knowledge of mitochondrial proteins contributing to synaptic transmission.

Structure of the Trypanosoma brucei p22 protein, a cytochrome oxidase subunit II-specific RNA-editing accessory factor

Kinetoplastid RNA (k-RNA) editing is a complex process in the mitochondria of kinetoplastid protozoa, including Trypanosoma brucei, that involves the guide RNA-directed insertion and deletion of uridines from precursor-mRNAs to produce mature, translatable mRNAs. k-RNA editing is performed by multiprotein complexes called editosomes. Additional non-editosome components termed k-RNA-editing accessory factors affect the extent of editing of specific RNAs or classes of RNAs. The T. brucei p22 protein was identified as one such accessory factor. Here we show that p22 contributes to cell growth in the procyclic form of T. brucei and functions as a cytochrome oxidase subunit II-specific k-RNA-editing accessory factor. To gain insight into its functions, we solved the crystal structure of the T. brucei p22 protein to 2.0-A resolution. The p22 structure consists of a six-stranded, antiparallel beta-sheet flanked by five alpha-helices. Three p22 subunits combine to form a tight trimer that is primarily stabilized by interactions between helical residues. One side of the trimer is strikingly acidic, while the opposite face is more neutral. Database searches show p22 is structurally similar to human p32, which has a number of functions, including regulation of RNA splicing. p32 interacts with a number of target proteins via its alpha1 N-terminal helix, which is among the most conserved regions between p22 and p32. Co-immunoprecipitation studies showed that p22 interacts with the editosome and the k-RNA accessory protein, TbRGG2, and alpha1 of p22 was shown to be important for the p22-TbRGG2 interaction. Thus, these combined studies suggest that p22 mediates its role in k-RNA editing by acting as an adaptor protein.

Role of the receptor for the globular domain of C1q protein in the pathogenesis of hepatitis C virus-related cryoglobulin vascular damage

Mixed cryoglobulinemia (MC) is a lymphoproliferative disorder observed in approximately 10 to 15% of hepatitis C virus (HCV)-infected patients. Circulating, nonenveloped HCV core protein, which has been detected in cryoprecipitable immune complexes, interacts with immunocytes through the receptor for the globular domain of C1q protein (gC1q-R). In this study, we have evaluated circulating gC1q-R levels in chronically HCV-infected patients, with and without MC. These levels were significantly higher in MC patients than in those without MC and in healthy controls and paralleled specific mRNA expression in PBL. Soluble gC1q-R circulates as a complexed form containing both C1q and HCV core proteins. Higher serum gC1q-R levels negatively correlated with circulating concentrations of the C4d fragment. The presence of sequestered C4d in the vascular bed of skin biopsies from MC patients was indicative of in situ complement activation. In vitro studies showed that release of soluble gC1q-R is regulated by HCV core-mediated inhibition of cell proliferation. Our results indicate that up-regulation of gC1q-R expression is a distinctive feature of MC, and that dysregulated shedding of C1q-R molecules contributes to vascular cryoglobulin-induced damage via the classic complement-mediated pathway.

Role of complement in immune regulation and its exploitation by virus

Complement is activated during the early phase of viral infection and promotes destruction of virus particles as well as the initiation of inflammatory responses. Recently, complement and complement receptors have been reported to play an important role in the regulation of innate as well as adaptive immune responses during infection. The regulation of host immune responses by complement involves modulation of dendritic cell activity in addition to direct effects on T-cell function. Intriguingly, many viruses encode homologs of complement regulatory molecules or proteins that interact with complement receptors on antigen-presenting cells and lymphocytes. The evolution of viral mechanisms to alter complement function may augment pathogen persistence and limit immune-mediated tissue destruction. These observations suggest that complement may play an important role in both innate and adaptive immune responses to infection as well as virus-mediated modulation of host immunity.

The autophagic inducer smARF interacts with and is stabilized by the mitochondrial p32 protein

The alternative reading frame (ARF) mRNA encodes two pro-death proteins, the nucleolar p19ARF and a shorter mitochondrial isoform, named smARF (hsmARF in human). While p19ARF can inhibit cell growth by causing cell cycle arrest or type I apoptotic cell death, smARF is able to induce type II autophagic cell death. Inappropriate proliferative signals generated by proto-oncogenes, such as c-Myc and E2F1, can elevate both p19ARF and smARF proteins. Here, we reveal a novel means of regulation of smARF protein steady state levels through its interactions with the mitochondrial p32. The p32 protein physically interacts with both human and murine smARF, and colocalizes with these short isoforms to the mitochondria. Remarkably, knocking down p32 protein levels significantly reduced the steady state levels of smARF by increasing its turn over. As a consequence, the ability of ectopically expressed smARF to induce autophagy and to cause mitochondrial membrane dissipation was significantly reduced. In contrast, the protein levels of full-length p19ARF, which mainly resides in the nucleolus, were not influenced by p32 depletion, suggesting that p32 exclusively stabilizes the mitochondrial smARF protein. Thus the interaction with p32 provides a means of specifically regulating the expression of the recently identified autophagic inducer, smARF, and adds yet another layer of complexity to the multifaceted regulation of the ARF gene.

Landscape of the hnRNP K protein-protein interactome

The heterogeneous nuclear ribonucleoprotein K is an ancient RNA/DNA-binding protein that is involved in multiple processes that compose gene expression. The pleiotropic action of K protein reflects its ability to interact with different classes of factors, interactions that are regulated by extracellular signals. We used affinity purification and MS to better define the repertoire of K protein partners. We identified a large number of new K protein partners, some typically found in subcellular compartments, such as plasma membrane, where K protein has not previously been seen. Electron microscopy showed K protein in the nucleus, cytoplasm, mitochondria, and in vicinity of plasma membrane. These observations greatly expanded the view of the landscape of K protein-protein interaction and provide new opportunities to explore signal transduction and gene expression in several subcellular compartments.

Acetylated Tat regulates human immunodeficiency virus type 1 splicing through its interaction with the splicing regulator p32

The human immunodeficiency virus type 1 (HIV-1) potent transactivator Tat protein mediates pleiotropic effects on various cell functions. Posttranslational modification of Tat affects its activity during viral transcription. Tat binds to TAR and subsequently becomes acetylated on lysine residues by histone acetyltransferases. Novel protein-protein interaction domains on acetylated Tat are then established, which are necessary for both sustained transcriptional activation of the HIV-1 promoter and viral transcription elongation. In this study, we investigated the identity of proteins that preferentially bound acetylated Tat. Using a proteomic approach, we identified a number of proteins that preferentially bound AcTat, among which p32, a cofactor of splicing factor ASF/SF-2, was identified. We found that p32 was recruited to the HIV-1 genome, suggesting a mechanism by which acetylation of Tat may inhibit HIV-1 splicing needed for the production of full-length transcripts. Using Tat from different clades, harboring a different number of acetylation sites, as well as Tat mutated at lysine residues, we demonstrated that Tat acetylation affected splicing in vivo. Finally, using confocal microscopy, we found that p32 and Tat colocalize in vivo in HIV-1-infected cells.

Interactions between rubella virus capsid and host protein p32 are important for virus replication

The distribution and morphology of mitochondria are dramatically affected during infection with rubella virus (RV). Expression of the capsid, in the absence of other viral proteins, was found to induce both perinuclear clustering of mitochondria and the formation of electron-dense intermitochondrial plaques, both hallmarks of RV-infected cells. We previously identified p32, a host cell mitochondrial matrix protein, as a capsid-binding protein. Here, we show that two clusters of arginine residues within capsid are required for stable binding to p32. Mutagenic ablation of the p32-binding site in capsid resulted in decreased mitochondrial clustering, indicating that interactions with this cellular protein are required for capsid-dependent reorganization of mitochondria. Recombinant viruses encoding arginine-to-alanine mutations in the p32-binding region of capsid exhibited altered plaque morphology and replicated to lower titers. Further analysis indicated that disruption of stable interactions between capsid and p32 was associated with decreased production of subgenomic RNA and, consequently, infected cells produced significantly lower amounts of viral structural proteins under these conditions. Together, these results suggest that capsid-p32 interactions are important for nonstructural functions of capsid that include regulation of virus RNA replication and reorganization of mitochondria during infection.

Cellular p32 recruits cytomegalovirus kinase pUL97 to redistribute the nuclear lamina

Replication of human cytomegalovirus is limited at the level of nucleocytoplasmic transport of viral capsids, a process that requires the disassembly of the nuclear lamina. Deletion of the protein kinase gene UL97 from the viral genome showed that the activity of pUL97 plays an important role for viral capsid egress. Here, we report that p32, a novel cellular interactor of the viral kinase pUL97, promotes the accumulation of pUL97 at the nuclear membrane by recruiting the p32-pUL97 complex to the lamin B receptor. Transfection of active pUL97, but not a catalytically inactive mutant, induced a redistribution of lamina components as demonstrated for recombinant lamin B receptor-green fluorescent protein and endogenous lamins A and C. Consistent with this, p32 itself and lamins were phosphorylated by pUL97. Importantly, overexpression of p32 in human cytomegalovirus-infected cells resulted in increased efficiency of viral replication and release of viral particles. Thus, it is highly suggestive that the cellular protein p32 recruits pUL97 to induce a dissolution of the nuclear lamina thereby facilitating the nuclear export of viral capsids.

Truncated variants of hyaluronan-binding protein 1 bind hyaluronan and induce identical morphological aberrations in COS-1 cells

Hyaluronan (HA)-binding protein 1 (HABP1) is multifunctional in nature and exists as a trimer through coiled-coil interaction between alpha-helices at its N- and C-termini. To investigate the importance of trimeric assemblage and HA-binding ability of HABP1, we generated and overexpressed variants of HABP1 by truncating the alpha-helices at its termini. Subsequently, these variants were transiently expressed in COS-1 cells to examine the influence of these structural variations on normal cell morphology, as compared with those imparted by HABP1. Substantiating the centrality of coiled-coil interaction for maintaining the trimeric assembly of HABP1, we demonstrate that disruption of trimerization does not alter the affinity of variants towards its ligand HA. Transient expression of HABP1 altered the morphology of COS-1 cells by generating numerous cytoplasmic vacuoles along with disruption of the f-actin network. Interestingly, the truncated variants also imparted identical morphological changes. Characterization of the cytoplasmic vacuoles revealed that most of these vacuoles were autophagic in nature, resembling those generated under stress conditions. The identical morphological changes manifested in COS-1 cells on transient expression of HABP1 or its variants is attributed to their comparable HA-binding ability, which in concert with endogenous HABP1, may deplete the cellular HA pool. Such quenching of HA below a threshold level in the cellular milieu could generate a stress condition, manifested through cytoplasmic vacuoles and a disassembly of the f-actin network.

The Ustilago maydis a2 mating-type locus genes lga2 and rga2 compromise pathogenicity in the absence of the mitochondrial p32 family protein Mrb1

The Ustilago maydis mrb1 gene specifies a mitochondrial matrix protein with significant similarity to mitochondrial p32 family proteins known from human and many other eukaryotic species. Compatible mrb1 mutant strains were able to mate and form dikaryotic hyphae; however, proliferation within infected tissue and the ability to induce tumor development of infected maize (Zea mays) plants were drastically impaired. Surprisingly, manifestation of the mrb1 mutant phenotype selectively depended on the a2 mating type locus. The a2 locus contains, in addition to pheromone signaling components, the genes lga2 and rga2 of unknown function. Deletion of lga2 in an a2Deltamrb1 strain fully restored pathogenicity, whereas pathogenicity was partially regained in an a2Deltamrb1Deltarga2 strain, implicating a concerted action between Lga2 and Rga2 in compromising pathogenicity in Deltamrb1 strains. Lga2 and Rga2 localized to mitochondria and Mrb1 interacted with Rga2 in the yeast two-hybrid system. Conditional expression of lga2 in haploid cells reduced vegetative growth, conferred mitochondrial fragmentation and mitochondrial DNA degradation, and interfered with respiratory activity. The consequences of lga2 overexpression depended on the expression strength and were greatly exacerbated in Deltamrb1 mutants. We propose that Lga2 interferes with mitochondrial fusion and that Mrb1 controls this activity, emphasizing a critical link between mitochondrial morphology and pathogenicity.

The Herpesvirus Saimiri open reading frame 73 gene product interacts with the cellular protein p32

The role of the gamma-2 herpesvirus open reading frame (ORF) 73 gene product has become the focus of considerable interest. It has recently been shown that the Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA) is expressed during a latent infection and can modulate both viral and cellular gene expression. The herpesvirus saimiri (HVS) ORF 73 gene product has some sequence homology to LANA; however, the role of HVS ORF 73 is unknown. We have previously demonstrated that HVS ORF73 is expressed in a stably transduced human carcinoma cell line, where HVS genomes persist as nonintegrated circular episomes. This implies that there may be some functional homology between these proteins. To further investigate the role of the HVS ORF 73 protein, the yeast two-hybrid system was employed to identify interacting cellular proteins. We demonstrate that ORF 73 interacts with the cellular protein p32 and triggers the accumulation of p32 in the nucleus. Using reporter gene-based transient-transfection assays, we demonstrate that ORF 73 can transactivate a number of heterologous promoter constructs and also upregulate its own promoter. Moreover, ORF 73 and p32 act synergistically to transactivate these promoters. The binding of ORF 73 to p32 is mediated by an amino-terminal arginine-rich domain, which contains two functionally distinct nuclear localization signals. The p32 binding domains are required for ORF 73 transactivating abilities and for ORF 73 to induce nuclear accumulation of p32. These results suggest that ORF 73 can function as a regulator of gene expression and that p32 is involved in ORF 73-dependent transcriptional activation.

Evidence for the presence of HABP1 pseudogene in multiple locations of mammalian genome

The gene encoding hyaluronan-binding protein 1 (HABP1) is expressed ubiquitously in different rat tissues, and is present in eukaryotic species from yeast to humans. Fluorescence in situ hybridization indicates that this is localized in human chromosome 17p13.3. Here, we report the presence of homologous sequences of HABP1 cDNA, termed processed HABP1 pseudogene in humans. This is concluded from an additional PCR product of ~0.5 kb, along with the expected band at approximately 5 kb as observed by PCR amplification of human genomic DNA with HABP1-specific primers. Partial sequencing of the 5-kb PCR product and comparison of the HABP1 cDNA with the sequence obtained from Genbank accession number AC004148 indicated that the HABP1 gene is comprised of six exons and five introns. The 0.5-kb additional PCR product was confirmed to be homologous to HABP1 cDNA by southern hybridization, sequencing, and by a sequence homology search. Search analysis with HABP1 cDNA sequence further revealed the presence of similar sequence in chromosomes 21 and 11, which could generate ~0.5 kb with the primers used. In this report, we describe the presence of several copies of the pseudogene of HABP1 spread over different chromosomes that vary in length and similarity to the HABP1 cDNA sequence. These are 1013 bp in chromosome 21 with 85.4% similarity, 1071 bp in chromosome 11 with 87.2% similarity, 818 bp in chromosome 15 with 82.3% similarity, and 323 bp in chromosome 4 with 84% similarity to HABP1 cDNA. We have also identified similar HABP1 pseudogenes in the rat and mouse genome. The human pseudogene sequence of HABP1 possesses a 10 base pair direct repeat of "AGAAAAATAA" in chromosome 21, a 12-bp direct repeat of "AG/CAAATTA/CAA/TTA" in chromosome 4, a 8-bp direct repeat of "ACAAAG/TCT" in chromosome 15. In the case of chromosome 11, there is an inverted repeat of "AGCCTGGGCGACAGAGCGAGA" ~50 bp upstream of the HABP1 pseudogene sequence. All of the HABP1 pseudogene sequences lack 5' promoter sequence and possess multiple mutations leading to the insertion of premature stop codons in all three reading frames. Rat and mouse homologs of the HABP1 pseudogene also contain multiple mutations, leading to the insertion of premature stop codons confirming the identity of a processed pseudogene.

Hyaluronan binding protein 1 (HABP1)/C1QBP/p32 is an endogenous substrate for MAP kinase and is translocated to the nucleus upon mitogenic stimulation

The role of hyaluronan binding protein 1 (HABP1) in cell signaling was investigated and in vitro kinase assay demonstrated that it is a substrate for MAP kinase. Phosphorylation of endogenous HABP1 was also observed following treatment of J774 cells with PMA. HABP1 was coimmunoprecipitated with activated ERK, confirming their physical interaction in the cellular context. Upon PMA stimulation of normal rat fibroblast (F111) and transformed (HeLa) cells, the HABP1 level in the cytoplasm gradually decreased with a parallel increase in the nucleus. In HeLa cells, within 6 h of PMA treatment, HABP1 was completely translocated to the nucleus, which was prevented by PD98059, a selective inhibitor of ERK. We also observed that the nuclear translocation of HABP1 is concurrent with that of ERK, suggesting that ERK activation is a requirement for the translocation of HABP1. It is thus established for the first time that HABP1 is a substrate for ERK and an integral part of the MAP kinase cascade.

The trypanosome homolog of human p32 interacts with RBP16 and stimulates its gRNA binding activity

RBP16 is a guide RNA (gRNA)-binding protein that was shown through immunoprecipitation experiments to interact with approximately 30% of total gRNAs in Trypanosoma brucei mitochondria. To gain insight into the biochemical function of RBP16, we used affinity chromatography and immunoprecipitation to identify RBP16 protein binding partners. By these methods, RBP16 does not appear to stably interact with the core editing machinery. However, fractionation of mitochondrial extracts on MBP-RBP16 affinity columns consistently isolated proteins of 12, 16, 18 and 22 kDa that were absent from MBP control columns. We describe here our analysis of one RBP16-associated protein, p22. The predicted p22 protein has significant sequence similarity to a family of multimeric, acidic proteins that includes human p32 and Saccharomyces cerevisiae mam33p. Glutaraldehyde crosslinking of recombinant p22 identified homo-multimeric forms of the protein, further substantiating its homology to p32. We confirmed the p22-RBP16 interaction and demonstrated that the two proteins bind each other directly by ELISA utilizing recombinant p22 and RBP16. p32 family members have been reported to modulate viral and cellular pre-mRNA splicing, in some cases by perturbing interaction of their binding partners with RNA. To determine whether p22 similarly affects the gRNA binding properties of RBP16, we titrated recombinant p22 into UV crosslinking assays. These experiments revealed that p22 significantly stimulates the gRNA binding capacity of RBP16. Thus, p22 has the potential to be a regulatory factor in T.brucei mitochondrial gene expression by modulating the RNA binding properties of RBP16.

Coordinate effects of human immunodeficiency virus type 1 protein Tat and cellular protein Puralpha on DNA replication initiated at the JC virus origin

JC virus (JCV) causes progressive multifocal leukoencephalopathy, a demyelinating disease in brains of individuals with AIDS. Previous work has shown that the Tat protein, encoded by human immunodeficiency virus type 1 (HIV-1), can interact with cellular protein Puralpha to enhance both TAR-dependent HIV-1 transcription and JCV late gene transcription. Tat has been shown to activate JCV transcription through interaction with Puralpha, which binds to promoter sequence elements near the JCV origin of replication. DNA footprinting has shown that Puralpha and large T-antigen cooperatively interact at several binding sites in the origin and transcriptional control region. Overexpression of Puralpha inhibits replication initiated at the JCV origin by T-antigen. In transfected glial cells Tat reversed this inhibition and enhanced DNA replication. In an in vitro replication system maximal activation by Tat, more than sixfold the levels achieved with T-antigen alone, was achieved in the presence of Puralpha. Effects of mutant Tat proteins on both activation of replication and binding to Puralpha have revealed that Cys22 exerts a conformational effect that affects both activities. The origin of an archetypal strain of JCV was less susceptible to activation of replication by Tat relative to the rearranged Mad-1 strain. These results have revealed a previously undocumented role for Tat in DNA replication and have indicated a regulatory role for JCV origin auxiliary sequences in replication and activation by Tat.

Retargeting of the mitochondrial protein p32/gC1Qr to a cytoplasmic compartment and the cell surface

p32/gC1qR is a small acidic protein that has been reported to have a broad range of distinct functions and to associate with a wide array of cellular, viral and bacterial proteins. It has been found in each of the main cellular compartments including mitochondria, nucleus and cytoplasm and is also thought to be located at the plasma membrane and secreted into the extracellular matrix. The true physiological role(s) of p32 remains controversial because it has been difficult to reconcile all of the findings on protein interactions and the seemingly disparate observations on compartmentalisation. However, it has been proposed that p32 is somehow involved in transport processes connecting diverse cellular compartments and the cell surface. Here we show that native p32 appears to be localised mainly in the mitochondria and is not detectable on the cell surface. However, addition of a short tag to the N-terminus of p32 appears to block its mitochondrial targeting, resulting in redirection into a cytoplasmic vesicular pattern, overlapping with the endoplasmic reticulum. The redirection of p32 results in an alteration in and co-localisation with ER markers including calreticulin, a lumenal ER chaperone. Furthermore, we show both by immunofluorescence and cross-linking studies that this also results in cell-surface expression of p32. These results indicate that, at least under certain circumstances, p32 can be retargeted and may help to provide an explanation for the diverse observations on its localization.

Mitochondrial protein p32 can accumulate in the nucleus

Human p32 was first isolated associated with the splicing factor ASF/SF-2. The p32 protein is translated as pre-protein from which a mitochondrial import signal is cleaved off to create the mature p32. The majority of p32 is consequently found in the mitochondria. In this study we investigated extramitochondrial p32. An increased nuclear localisation of endogenous p32 was demonstrated as a response to leptomycin B or actinomycin D treatment of cells. Mature p32 gene and deletion mutants were cloned into enhanced green fluorescence protein reporter plasmids. On transfection, EGFP-p32 protein was mainly localised to the cytoplasm and to a lesser extent to the nucleus of transfected COS cells. Upon treatment with actinomycin D or leptomycin B, the EGFP-p32 protein accumulated in the nucleus. Deletion analysis indicated which regions of EGFP-p32 are involved in nuclear export and nuclear import.

Interaction between herpes simplex virus type 1 IE63 protein and cellular protein p32

The herpes simplex virus type 1 (HSV-1) immediate-early gene IE63 (ICP27), the only HSV-1 regulatory gene with a homologue in every mammalian and avian herpesvirus sequenced so far, is a multifunctional protein which regulates transcriptional and posttranscriptional processes. One of its posttranscriptional effects is the inhibition of splicing of viral and cellular transcripts. We previously identified heterogeneous nuclear ribonucleoprotein (hnRNP) K and casein kinase 2 (CK2) as two protein partners of IE63 (H. Bryant et al., J. Biol. Chem. 274:28991-28998, 1999). Here, using a yeast two-hybrid assay, we identify another partner of IE63, the cellular protein p32. Confirmation of this interaction was provided by coimmunoprecipitation from virus-infected cells and recombinant p32 binding assays. A p32-hnRNP K-CK2 complex, which required IE63 to form, was isolated from HSV-1-infected cells, and coimmunoprecipitating p32 was phosphorylated by CK2. Expression of IE63 altered the cytoplasmic distribution of p32, with some now colocalizing with IE63 in the nuclei of infected and transfected cells. As p32 copurifies with splicing factors and can inhibit splicing, we propose that IE63 together with p32, possibly with other IE63 partner proteins, acts to disrupt or regulate pre-mRNA splicing. As well as contributing to host cell shutoff, this effect could facilitate splicing-independent nuclear export of viral transcripts.

Physical association between the EBV protein EBNA-1 and P32/TAP/hyaluronectin

Epstein-Barr virus (EBV) nuclear antigen-1 (EBNA-1) is a protein expressed constitutively during EBV latency. It is required to support the replication of the EBV genome once per cell cycle via the latent origin of replication, oriP. EBNA-1 also can activate transcription through binding to the family repeats of oriP. We wished to identify candidate cellular protein(s) that may interact with EBNA-1 and mediate these functions. A 32-kd protein was co-immunoprecipitated with EBNA-1 from 293 cells using a monoclonal antibody EBNA.OT1x. The regions of EBNA-1 which interact with this protein were studied using two deletion clones and mapped to EBNA-1 residues 1-102 and 325-357. Deletion of this region was shown previously in a mutant of EBNA-1 which had dominant-negative effects on both DNA replication and transactivation assays. The 32-kd protein was found to react with a polyclonal antiserum against P32/TAP (HIV Tat associated protein), which is known to interact with other RNA binding proteins and the RNA splicing factor SF2. The function of P32 was therefore proposed to involve RNA processing. In addition, this molecule was recently identified as hyaluronectin, which binds hyaluronic acid. Because several reports documented that intracellular hyaluronic acid can potentially affect cell proliferation, the association between EBNA-1 and P32/TAP/hyaluronectin may help the maintenance of episomal viral DNA within proliferating cells.

Analysis of HIV-1 Tat effects in Xenopus laevis embryos

Tat is one of the regulatory proteins of the HIV-1 virus. To date, besides the transactivation activity, a myriad of effects exerted by HIV-1 Tat on cellular and viral genes have been observed. The present study investigated the in vivo effects of HIV-1 Tat protein in the Xenopus embryo. We adopted the Xenopus system since expression of putative regulatory factors in the embryo has been widely used as a quick and effective first screen for protein function. Xenopus' early development is well characterized by stage-specific phenotypes, therefore, an in vivo HIV-1 Tat-mediated aberrant phenotype can easily be detected and analyzed. HIV-1 Tat protein expression through injection of synthetic mRNA into zygotes produced a marked delay in gastrulation leading to altered specification of the anterior-posterior axis and to partial or total loss of anterior structures. HIV-1 Tat effects resulted in a general suppression of gene expression, including that of Xbra and gsc, two early genes whose expression is required for proper gastrulation. The specificity of Tat effects was demonstrated by injecting a 'loss of function' mutant (Tat-C37S), lacking a single cysteine residue, which did not yield any effect. Both Tat and Tat-C37S were found to be localized mainly in the nucleus. The importance of subcellular targeting for the effects caused by HIV-1 Tat was demonstrated by injecting a second mutant (Tat-BDM), carrying an altered nuclear localization signal sequence. The Tat-BDM protein localized in the cytoplasm and accumulated at the cell membrane. Embryos injected with Tat-BDM mRNA did not develop beyond gastrulation. The importance of proper protein conformation and subcellular localization in determining Tat effects is discussed.

Human p32: a coactivator for Epstein-Barr virus nuclear antigen-1-mediated transcriptional activation and possible role in viral latent cycle DNA replication

The Epstein-Barr virus (EBV) nuclear antigen-1 (EBNA-1) is required for the maintenance of the viral chromosome in latently infected, proliferating cells and plays a role in latent cycle DNA replication. EBNA-1 also functions as a positive and negative regulator of EBV gene expression. We have investigated the interaction of EBNA-1 with p32, a host mitochondrial protein that associates with EBNA-1 in EBV-positive Burkitt's lymphoma cells. Using a chromatin immunoprecipitation assay, we found that a fraction of p32 localizes to the viral latent cycle origin of DNA replication oriP in vivo. p32 binds EBNA-1 independently of other proteins or DNA. EBNA-1 variants lacking one of two p32 binding elements did not interact stably with p32 in cultured cells and were defective for both transcriptional activation of a reporter gene linked to oriP FR and replication and/or maintenance of a plasmid bearing oriP. These results support a role for p32 in transcriptional activation by EBNA-1 and suggest that p32 plays a role in EBV latent cycle DNA replication.

Protein kinase C [micro] is regulated by the multifunctional chaperon protein p32

We identified the multifunctional chaperon protein p32 as a protein kinase C (PKC)-binding protein interacting with PKCalpha, PKCzeta, PKCdelta, and PKC mu. We have analyzed the interaction of PKC mu with p32 in detail, and we show here in vivo association of PKC mu, as revealed from yeast two-hybrid analysis, precipitation assays using glutathione S-transferase fusion proteins, and reciprocal coimmunoprecipitation. In SKW 6.4 cells, PKC mu is constitutively associated with p32 at mitochondrial membranes, evident from colocalization with cytochrome c. p32 interacts with PKC mu in a compartment-specific manner, as it can be coimmunoprecipitated mainly from the particulate and not from the soluble fraction, despite the presence of p32 in both fractions. Although p32 binds to the kinase domain of PKC mu, it does not serve as a substrate. Interestingly, PKC mu-p32 immunocomplexes precipitated from the particulate fraction of two distinct cell lines, SKW 6.4 and 293T, show no detectable substrate phosphorylation. In support of a kinase regulatory function of p32, addition of p32 to in vitro kinase assays blocked, in a dose-dependent manner, aldolase but not autophosphorylation of PKC mu, suggesting a steric hindrance of substrate within the kinase domain. Together, these findings identify p32 as a novel, compartment-specific regulator of PKC mu kinase activity.

Functional interaction between the HIV transactivator Tat and the transcriptional coactivator PC4 in T cells

The human immunodeficiency virus (HIV) transactivator Tat is a potent activator of transcription from the HIV long terminal repeat and is essential for efficient viral gene expression and replication. Tat has been shown to interact with components of the basal transcription machinery and transcriptional activators. Here we identify the cellular coactivator PC4 as a Tat-interacting protein using the yeast two-hybrid system and confirmed this interaction both in vitro and in vivo by coimmunoprecipitation. We found that this interaction has a functional outcome in that PC4 overexpression enhanced activation of the HIV long terminal repeat in transient transfection studies in a Tat-dependent manner. The domains of PC4 and Tat required for the interaction were mapped. In vitro binding studies showed that the basic transactivation-responsive binding domain of Tat is required for the interaction with PC4. The minimum region of PC4 required for Tat binding was amino acids 22-91, whereas mutation of the lysine-rich domain between amino acids 22 and 43 prevented interaction with Tat. Tat-PC4 interactions may be controlled by phosphorylation, because phosphorylation of PC4 by casein kinase II inhibited interactions with Tat both in vivo and in vitro. We propose that PC4 may be involved in linking Tat to the basal transcription machinery.

gC1q-R/p32, a C1q-binding protein, is a receptor for the InlB invasion protein of Listeria monocytogenes

InlB is a Listeria monocytogenes protein that promotes entry of the bacterium into mammalian cells by stimulating tyrosine phosphorylation of the adaptor proteins Gab1, Cbl and Shc, and activation of phosphatidyl- inositol (PI) 3-kinase. Using affinity chromatography and enzyme-linked immunosorbent assay, we demonstrate a direct interaction between InlB and the mammalian protein gC1q-R, the receptor of the globular part of the complement component C1q. Soluble C1q or anti-gC1q-R antibodies impair InlB-mediated entry. Transient transfection of GPC16 cells, which are non-permissive to InlB-mediated entry, with a plasmid-expressing human gC1q-R promotes entry of InlB-coated beads. Furthermore, several experiments indicate that membrane recruitment and activation of PI 3-kinase involve an InlB-gC1q-R interaction and that gC1q-R associates with Gab1 upon stimulation of Vero cells with InlB. Thus, gC1q-R constitutes a cellular receptor involved in InlB-mediated activation of PI 3-kinase and tyrosine phosphorylation of the adaptor protein Gab1. After E-cadherin, the receptor for internalin, gC1q-R is the second identified mammalian receptor promoting entry of L. monocytogenes into mammalian cells.

Mitochondrial proteins at unexpected cellular locations: export of proteins from mitochondria from an evolutionary perspective

Researchers in a wide variety of unrelated areas studying functions of different proteins are unexpectedly finding that their proteins of interest are actually mitochondrial proteins, although functions would appear to be extramitochondrial. We review the leading current examples of mitochondrial macromolecules indicated to be also present outside of mitochondria that apparently exit from mitochondria to arrive at their destinations. Mitochondrial chaperones, which have been implicated in growth and development, autoimmune diseases, cell mortality, antigen presentation, apoptosis, and resistance to antimitotic drugs, provide some of the best studied examples pointing to roles for mitochondria and mitochondrial proteins in diverse cellular phenomena. To explain the observations, we propose that specific export mechanisms exist by which certain proteins exit mitochondria, allowing these proteins to have additional functions at specific extramitochondrial sites. Several possible mechanisms by which mitochondrial proteins could be exported are discussed. Gram-negative proteobacteria, from which mitochondria evolved, contain a number of different mechanisms for protein export. It is likely that mitochondria either retained or evolved export mechanisms for certain specific proteins.

Interaction of the alpha(1B)-adrenergic receptor with gC1q-R, a multifunctional protein

gC1q-R, a multifunctional protein, was found to bind with the carboxyl-terminal cytoplasmic domain of the alpha(1B)-adrenergic receptor (173 amino acids, amino acids 344-516) in a yeast two-hybrid screen of a cDNA library prepared from the rat liver. In a series of studies with deletion mutants in this region, the ten arginine-rich amino acids (amino acids 369-378) were identified as the site of interaction. The interaction was confirmed by specific co-immunoprecipitation of gC1q-R with full-length alpha(1B)-adrenergic receptors expressed on transfected COS-7 cells, as well as by fluorescence confocal laser scanning microscopy, which showed co-localization of these proteins in intact cells. Interestingly, the alpha(1B)-adrenergic receptors were exclusively localized to the region of the plasma membrane in COS-7 cells that expressed the alpha(1B)-adrenergic receptor alone, whereas gC1q-R was localized in the cytoplasm in COS-7 cells that expressed gC1q-R alone; however, in cells that co-expressed alpha(1B)-adrenergic receptors and gC1q-R, most of the alpha(1B)-adrenergic receptors were co-localized with gC1q-R in the intracellular region, and a remarkable down-regulation of receptor expression was observed. These observations suggest a new role for the previously identified complement regulatory molecule, gC1q-R, in regulating the cellular localization and expression of the alpha(1B)-adrenergic receptors.

Crystal structure of human p32, a doughnut-shaped acidic mitochondrial matrix protein

Human p32 (also known as SF2-associated p32, p32/TAP, and gC1qR) is a conserved eukaryotic protein that localizes predominantly in the mitochondrial matrix. It is thought to be involved in mitochondrial oxidative phosphorylation and in nucleus-mitochondrion interactions. We report the crystal structure of p32 determined at 2.25 A resolution. The structure reveals that p32 adopts a novel fold with seven consecutive antiparallel beta-strands flanked by one N-terminal and two C-terminal alpha-helices. Three monomers form a doughnut-shaped quaternary structure with an unusually asymmetric charge distribution on the surface. The implications of the structure on previously proposed functions of p32 are discussed and new specific functional properties are suggested.

Epithelin/granulin growth factors: extracellular cofactors for HIV-1 and HIV-2 Tat proteins

Epithelin/granulin growth factor is synthesized as a 593 amino acid precursor protein that contains 7.5 imperfectly conserved repeats of approximately 57 amino acids. Processed epithelin/granulin peptides have been isolated from vertebrate/invertebrate species and are growth factors implicated in epithelial and haemic cell function. Here they are identified as Human Immunodeficiency Virus (HIV) Tat binding proteins using the yeast two-hybrid assay. Intracellularly in yeast, mutation of selected cysteines in an epithelin/granulin dimeric repeat caused loss of binding to Tat exon 1. In vitro binding of HIV-1 and HIV-2 Tat to epithelin/granulin dimeric and monomeric repeats was also observed by GST-glutathione bead "pulldown" assays. Because Tat is actively secreted from HIV-infected cells and has been shown to serve as a mitogenic factor for angiogenesis and for Kaposi-like cells, our observations suggest that epithelin/granulin growth factors may function as biologically important extracellular Tat co-factors.

Activation of integrated provirus requires histone acetyltransferase. p300 and P/CAF are coactivators for HIV-1 Tat

A unique aspect of the retrovirus life cycle is the obligatory integration of the provirus into host cell chromosomes. Unlike viruses that do not integrate, retroviruses must conserve an ability to activate transcription from a chromatin context. Human immunodeficiency virus (HIV)-1 encodes an unusual and an unusually potent transcriptional transactivator, Tat, which binds to a nascent viral leader RNA, TAR. The action of Tat has been well studied in various reductive model systems; however, the physiological mechanism through which Tat gains access to chromatin-associated proviral long terminal repeats (LTRs) is not understood. We show here that a nuclear histone acetyltransferase activity associates with Tat. Intracellularly, we found that Tat forms a ternary complex with p300 and P/CAF, two histone acetyltransferases (HATs). A murine cell defect in Tat transactivation of the HIV-1 LTR was linked to the reduced abundance of p300 and P/CAF. Thus, overexpression of p300 and P/CAF reconstituted Tat transactivation of the HIV-1 LTR in NIH3T3 cells to a level similar to that observed for human cells. By using transdominant p300 or P/CAF mutants that lack enzymatic activity, we delineated a requirement for the HAT component from the latter but not the former in Tat function. Finally, we observed that Tat-associated HAT is preferentially important for transactivation of integrated, but not unintegrated, HIV-1 LTR.

The Multiligand-Binding Protein gC1qR, Putative C1q Receptor, Is a Mitochondrial Protein

A protein of 33 kDa (p33) that tightly binds to the globular domains of the first complement component, C1q, is thought to serve as the major C1q receptor (gC1qR) on B cells, neutrophils, and mast cells. However, the cellular routing and the subcellular localization of p33/gC1qR are unknown. We have performed confocal laser-scanning microscopy and found that p33/gC1qR is present in intracellular compartments, where it colocalizes with the mitochondrial marker protein, pyruvate dehydrogenase. No surface staining for p33/gC1qR on endothelial EA.hy926 cells was observed. A fusion protein of the p33/gC1qR presequence with green fluorescent protein translocated to the mitochondria of transfected COS-7 cells. Concomitantly, a 6-kDa portion of the fusion protein was proteolytically removed. The 33 amino-terminal residues of the presequence proved sufficient to direct reporter constructs to mitochondria. Association of p33/gC1qR with mitoplasts indicated that the mature protein of 209 residues resides in the matrix and/or the inner membrane of mitochondria. Immunocytochemistry of fetal mice tissues revealed a ubiquitous expression of p33/gC1qR, most prominently in tissues that are rich in mitochondria. Thus, the candidate complement receptor p33/gC1qR of intact cells cannot interact with plasma C1q due to mutually exclusive localizations of the components. The functional role of p33/gC1qR needs to be reconsidered.