Liquid-Liquid Phase Transition Drives Intra-chloroplast Cargo Sorting.

In eukaryotic cells, organelle biogenesis is pivotal for cellular function and cell survival. Chloroplasts are unique organelles with a complex internal membrane network. The mechanisms of the migration of imported nuclear-encoded chloroplast proteins across the crowded stroma to thylakoid membranes are less understood. Here, we identified two Arabidopsis ankyrin-repeat proteins, STT1 and STT2, that specifically mediate sorting of chloroplast twin arginine translocation (cpTat) pathway proteins to thylakoid membranes. STT1 and STT2 form a unique hetero-dimer through interaction of their C-terminal ankyrin domains. Binding of cpTat substrate by N-terminal intrinsically disordered regions of STT complex induces liquid-liquid phase separation. The multivalent nature of STT oligomer is critical for phase separation. STT-Hcf106 interactions reverse phase separation and facilitate cargo targeting and translocation across thylakoid membranes. Thus, the formation of phase-separated droplets emerges as a novel mechanism of intra-chloroplast cargo sorting. Our findings highlight a conserved mechanism of phase separation in regulating organelle biogenesis.

Translational control of the activation of transcription factor NF-κB and production of type I interferon by phosphorylation of the translation factor eIF4E.

Type I interferon is an integral component of the antiviral response, and its production is tightly controlled at the levels of transcription and translation. The eukaryotic translation-initiation factor eIF4E is a rate-limiting factor whose activity is regulated by phosphorylation of Ser209. Here we found that mice and fibroblasts in which eIF4E cannot be phosphorylated were less susceptible to virus infection. More production of type I interferon, resulting from less translation of Nfkbia mRNA (which encodes the inhibitor IκBα), largely explained this phenotype. The lower abundance of IκBα resulted in enhanced activity of the transcription factor NF-κB, which promoted the production of interferon-ß (IFN-ß). Thus, regulated phosphorylation of eIF4E has a key role in antiviral host defense by selectively controlling the translation of an mRNA that encodes a critical suppressor of the innate antiviral response.

The translocon protein FtsHi1 is an ATP-dependent DNA/RNA helicase that prevents R-loop accumulation in chloroplasts.

R-loops, three-stranded nucleic acid structures consisting of a DNA: RNA hybrid and displaced single-stranded DNA, play critical roles in gene expression and genome stability. How R-loop homeostasis is integrated into chloroplast gene expression remains largely unknown. We found an unexpected function of FtsHi1, an inner envelope membrane-bound AAA-ATPase in chloroplast R-loop homeostasis of Arabidopsis thaliana. Previously, this protein was shown to function as a component of the import motor complex for nuclear-encoded chloroplast proteins. However, this study provides evidence that FtsHi1 is an ATP-dependent helicase that efficiently unwinds both DNA-DNA and DNA-RNA duplexes, thereby preventing R-loop accumulation. Over-accumulation of R-loops could impair chloroplast transcription but not necessarily genome integrity. The dual function of FtsHi1 in both protein import and chloroplast gene expression may be important to coordinate the biogenesis of nuclear- and chloroplast-encoded subunits of multi-protein photosynthetic complexes. This study suggests a mechanical link between protein import and R-loop homeostasis in chloroplasts of higher plants.

Functional redox links between lumen thiol oxidoreductase1 and serine/threonine-protein kinase STN7.

In response to changing light quantity and quality, photosynthetic organisms perform state transitions, a process which optimizes photosynthetic yield and mitigates photo-damage. The serine/threonine-protein kinase STN7 phosphorylates the light-harvesting complex of photosystem II (PSII; light-harvesting complex II), which then migrates from PSII to photosystem I (PSI), thereby rebalancing the light excitation energy between the photosystems and restoring the redox poise of the photosynthetic electron transport chain. Two conserved cysteines forming intra- or intermolecular disulfide bonds in the lumenal domain (LD) of STN7 are essential for the kinase activity although it is still unknown how activation of the kinase is regulated. In this study, we show lumen thiol oxidoreductase 1 (LTO1) is co-expressed with STN7 in Arabidopsis (Arabidopsis thaliana) and interacts with the LD of STN7 in vitro and in vivo. LTO1 contains thioredoxin (TRX)-like and vitamin K epoxide reductase domains which are related to the disulfide-bond formation system in bacteria. We further show that the TRX-like domain of LTO1 is able to oxidize the conserved lumenal cysteines of STN7 in vitro. In addition, loss of LTO1 affects the kinase activity of STN7 in Arabidopsis. Based on these results, we propose that LTO1 helps to maintain STN7 in an oxidized active state in state 2 through redox interactions between the lumenal cysteines of STN7 and LTO1.

ECD1 functions as an RNA-editing trans-factor of rps14-149 in plastids and is required for early chloroplast development in seedlings.

Chloroplast development is a highly complex process and the regulatory mechanisms have not yet been fully characterized. In this study, we identified Early Chloroplast Development 1 (ECD1), a chloroplast-localized pentatricopeptide repeat protein (PPR) belonging to the PLS subfamily. Inactivation of ECD1 in Arabidopsis led to embryo lethality, and abnormal embryogenesis occurred in ecd1/+ heterozygous plants. A decrease in ECD1 expression induced by RNAi resulted in seedlings with albino cotyledons but normal true leaves. The aberrant morphology and under-developed thylakoid membrane system in cotyledons of RNAi seedlings suggests a role of ECD1 specifically in chloroplast development in seedlings. In cotyledons of ECD1-RNAi plants, RNA-editing of rps14-149 (encoding ribosomal protein S14) was seriously impaired. In addition, dramatically decreased plastid-encoded RNA polymerase-dependent gene expression and abnormal chloroplast rRNA processing were also observed. Taken together, our results indicate that ECD1 is indispensable for chloroplast development at the seedling stage in Arabidopsis.

Translational control of the innate immune response through IRF-7.

Transcriptional activation of cytokines, such as type-I interferons (interferon (IFN)-alpha and IFN-beta), constitutes the first line of antiviral defence. Here we show that translational control is critical for induction of type-I IFN production. In mouse embryonic fibroblasts lacking the translational repressors 4E-BP1 and 4E-BP2, the threshold for eliciting type-I IFN production is lowered. Consequently, replication of encephalomyocarditis virus, vesicular stomatitis virus, influenza virus and Sindbis virus is markedly suppressed. Furthermore, mice with both 4E- and 4E-BP2 genes (also known as Eif4ebp1 and Eif4ebp2, respectively) knocked out are resistant to vesicular stomatitis virus infection, and this correlates with an enhanced type-I IFN production in plasmacytoid dendritic cells and the expression of IFN-regulated genes in the lungs. The enhanced type-I IFN response in 4E-BP1-/- 4E-BP2-/- double knockout mouse embryonic fibroblasts is caused by upregulation of interferon regulatory factor 7 (Irf7) messenger RNA translation. These findings highlight the role of 4E-BPs as negative regulators of type-I IFN production, via translational repression of Irf7 mRNA.

The immune regulation and therapeutic potential of the SMAD gene family in breast cancer.

Breast cancer is a serious threat to human health. The transforming growth factor-ß signaling pathway is an important pathway involved in the occurrence and development of cancer. The SMAD family genes are responsible for the TGF-ß signaling pathway. However, the mechanism by which genes of the SMAD family are involved in breast cancer is still unclear. Therefore, it is necessary to investigate the biological roles of the SMAD family genes in breast cancer. We downloaded the gene expression data, gene mutation data, and clinical pathological data of breast cancer patients from the UCSC Xena database. We used the Wilcox test to estimate the expression of genes of the SMAD family in cancers. And the biological functions of SMAD family genes using the DAVID website. The Pearson correlation method was used to explore the immune cell infiltration and drug response of SMAD family genes. We conducted in biological experiments vitro and vivo. In this study, we integrated the multi-omics data from TCGA breast cancer patients for analysis. The expression of genes of SMAD family was significantly dysregulated in patients with breast cancer. Except for SMAD6, the expression of other SMAD family genes was positively correlated. We also found that genes of the SMAD family were significantly enriched in the TGF-ß signaling pathway, Hippo signaling pathway, cell cycle, and cancer-related pathways. In addition, SMAD3, SMAD6, and SMAD7 were lowly expressed in stage II breast cancer, while SMAD4 and SMAD2 were lowly expressed in stage III cancer. Furthermore, the expression of genes of the SMAD family was significantly correlated with immune cell infiltration scores. Constructing a xenograft tumor mouse model, we found that SMAD3 knockdown significantly inhibited tumorigenesis. Finally, we analyzed the association between these genes and the IC50 value of drugs. Interestingly, patients with high expression of SMAD3 exhibited significant resistance to dasatinib and staurosporine, while high sensitivity to tamoxifen and auranofin. In addition, SMAD3 knockdown promoted the apoptosis of BT-549 cells and decreased cell activity, and BAY-1161909 and XK-469 increased drug efficacy. In conclusion, genes of the SMAD family play a crucial role in the development of breast cancer.

The N-terminal region of IFITM3 modulates its antiviral activity by regulating IFITM3 cellular localization.

Interferon-inducible transmembrane (IFITM) protein family members IFITM1, -2, and -3 restrict the infection of multiple enveloped viruses. Significant enrichment of a minor IFITM3 allele was recently reported for patients who were hospitalized for seasonal and 2009 H1N1 pandemic flu. This IFITM3 allele lacks the region corresponding to the first amino-terminal 21 amino acids and is unable to inhibit influenza A virus. In this study, we found that deleting this 21-amino-acid region relocates IFITM3 from the endosomal compartments to the cell periphery. This finding likely underlies the lost inhibition of influenza A virus that completes its entry exclusively within endosomes at low pH. Yet, wild-type IFITM3 and the mutant with the 21-amino-acid deletion inhibit HIV-1 replication equally well. Given the pH-independent nature of HIV-1 entry, our results suggest that IFITM3 can inhibit viruses that enter cells via different routes and that its N-terminal region is specifically required for controlling pH-dependent viruses.

eIF4E phosphorylation promotes tumorigenesis and is associated with prostate cancer progression.

Translational regulation plays a critical role in the control of cell growth and proliferation. A key player in translational control is eIF4E, the mRNA 5' cap-binding protein. Aberrant expression of eIF4E promotes tumorigenesis and has been implicated in cancer development and progression. The activity of eIF4E is dysregulated in cancer. Regulation of eIF4E is partly achieved through phosphorylation. However, the physiological significance of eIF4E phosphorylation in mammals is not clear. Here, we show that knock-in mice expressing a nonphosphorylatable form of eIF4E are resistant to tumorigenesis in a prostate cancer model. By using a genome-wide analysis of translated mRNAs, we show that the phosphorylation of eIF4E is required for translational up-regulation of several proteins implicated in tumorigenesis. Accordingly, increased phospho-eIF4E levels correlate with disease progression in patients with prostate cancer. Our findings establish eIF4E phosphorylation as a critical event in tumorigenesis. These findings raise the possibility that chemical compounds that prevent the phosphorylation of eIF4E could act as anticancer drugs.

The IFITM proteins inhibit HIV-1 infection.

Type I interferon protects cells from virus infection through the induction of a group of genes collectively named interferon-stimulated genes (ISGs). In this study, we utilized short hairpin RNA (shRNA) to deplete ISGs in SupT1 cells in order to identify ISGs that suppress the production of human immunodeficiency virus type 1 (HIV-1). Among the ISG candidates thus identified were interferon-induced transmembrane (IFITM) proteins, including IFITM1, IFITM2, and IFITM3, that potently inhibit HIV-1 replication at least partially through interfering with virus entry. Further mutagenesis analysis shows that the intracellular region, rather than the N- and C-terminal extracellular domains, is essential for the antiviral activity of IFITM1. Altogether, these data suggest that the IFITM proteins serve as important components of the innate immune system to restrict HIV-1 infection.

Control of eIF4E cellular localization by eIF4E-binding proteins, 4E-BPs.

Eukaryotic initiation factor (eIF) 4E, the mRNA 5'-cap-binding protein, mediates the association of eIF4F with the mRNA 5'-cap structure to stimulate cap-dependent translation initiation in the cytoplasm. The assembly of eIF4E into the eIF4F complex is negatively regulated through a family of repressor proteins, called the eIF4E-binding proteins (4E-BPs). eIF4E is also present in the nucleus, where it is thought to stimulate nuclear-cytoplasmic transport of certain mRNAs. eIF4E is transported to the nucleus via its interaction with 4E-T (4E-transporter), but it is unclear how it is retained in the nucleus. Here we show that a sizable fraction (approximately 30%) of 4E-BP1 is localized to the nucleus, where it binds eIF4E. In mouse embryo fibroblasts (MEFs) subjected to serum starvation and/or rapamycin treatment, nuclear 4E-BPs sequester eIF4E in the nucleus. A dramatic loss of nuclear 4E-BP1 occurs in c-Ha-Ras-expressing MEFs, which fail to show starvation-induced nuclear accumulation of eIF4E. Therefore, 4E-BP1 is a regulator of eIF4E cellular localization.

The transmembrane domain of BST-2 determines its sensitivity to down-modulation by human immunodeficiency virus type 1 Vpu.

Bone marrow stromal cell antigen 2 (BST-2, also known as tetherin) restricts the production of a number of enveloped viruses by blocking virus release from the cell surface. This antiviral activity is counteracted by such viral factors as Vpu of human immunodeficiency virus type 1 (HIV-1). Here, we report that Vpu antagonizes human BST-2 but not BST-2 derived from African green monkeys. The determinants of susceptibility to Vpu map to the transmembrane domain of BST-2. In accordance with this, expression of human BST-2 containing a modified transmembrane domain effectively blocks the replication of wild-type Vpu-expressing HIV-1 in CD4+ T cells. Furthermore, these BST-2 variants, as opposed to wild-type human BST-2, are refractory to Vpu-mediated down-regulation as a result of an attenuated interaction with Vpu. In view of the work by others pointing to a key role of the transmembrane domain of Vpu in promoting virus release, our data suggest that a direct interaction through the transmembrane domain of each of these two proteins is a prerequisite for Vpu to down-modulate BST-2.

Insulin-like growth factor II mRNA binding protein 1 associates with Gag protein of human immunodeficiency virus type 1, and its overexpression affects virus assembly.

The assembly of human immunodeficiency virus type 1 (HIV-1) particles is driven by viral Gag protein. This function of Gag not only benefits from its self-multimerization property but also depends on its interaction with a number of cellular factors such as TSG101 and ALIX/AIP1 that promote virus budding and release from cell surfaces. However, interaction with Gag also allows some cellular factors such as APOBEC3G and Trim5alpha to access viral replication machinery and block viral replication. In this study, we report a new HIV-1 Gag-binding factor named insulin-like growth factor II mRNA binding protein 1 (IMP1). Gag-IMP1 interaction requires the second zinc finger of the nucleocapsid (NC) domain of Gag and the KH3 and KH4 domains of IMP1. A fourfold reduction of HIV-1 infectivity was seen with overexpression of the wild-type IMP1 and its mutant that is able to interact with Gag but not with overexpression of IMP1 mutants exhibiting Gag-binding deficiency. The decreased viral infectivity was further shown as a result of diminished viral RNA packaging, abrogated Gag processing on the cellular membranes, and impeded maturation of virus particles. Together, these results demonstrate that IMP1 interacts with HIV-1 Gag protein and is able to block the formation of infectious HIV-1 particles.

eIF4E--from translation to transformation.

Over the years, studies have focused on the transcriptional regulation of oncogenesis. More recently, a growing emphasis has been placed on translational control. The Ras and Akt signal transduction pathways play a critical role in regulating mRNA translation and cellular transformation. The question arises: How might the Ras and Akt signaling pathways affect translation and mediate transformation? These pathways converge on a crucial effector of translation, the initiation factor eIF4E, which binds the 5'cap of mRNAs. This review focuses on the role of eIF4E in oncogenesis. eIF4E controls the translation of various malignancy-associated mRNAs which are involved in polyamine synthesis, cell cycle progression, activation of proto-oncogenes, angiogenesis, autocrine growth stimulation, cell survival, invasion and communication with the extracellular environment. eIF4E-mediated translational modulation of these mRNAs plays a pivotal role in both tumor formation and metastasis. Interestingly, eIF4E activity is implicated in mitosis, embryogenesis and in apoptosis. Finally, the finding that eIF4E is overexpressed in several human cancers makes it a prime target for anticancer therapies.

Fragile X mental retardation protein restricts replication of human immunodeficiency virus type 1.

Gag protein is the major structural component of human immunodeficiency virus type 1 (HIV-1) particles and drives virus assembly on cellular membranes. This function of Gag is attributed to its intrinsic self-multimerization ability as well as its interaction with cellular factors such as TSG101 that binds to the PTAP sequence in the p6 region of Gag and promotes HIV-1 budding through recruiting the ESCRT (endosomal sorting complex required for transport). As a result of its essential role in virus assembly, Gag also becomes the target of cellular restriction factors such as APOBEC3G and Trim5alpha. In this study, we report that the fragile X mental retardation protein (FMRP) interacts with HIV-1 Gag and is packaged into virus particles. Although knockdown of FMRP does not markedly affect production of virus particles, infectivity of HIV-1 virions was significantly decreased. Consistent with this observation, overexpression of the wild type FMRP, but not the FMRP mutants that lack the KH1 or the KH2 domains, led to 2- to 3-fold reduction of virus infectivity. Together, these results suggest that FMRP diminishes HIV-1 infectivity through association with viral Gag protein and virus particles.

Insulin-like growth factor II mRNA binding protein 1 modulates Rev-dependent human immunodeficiency virus type 1 RNA expression.

Human immunodeficiency virus type 1 (HIV-1) needs to overcome cellular counter mechanisms such as to successfully propagate itself. Results of our recent studies show that overexpression of insulin-like growth factor II mRNA binding protein 1 (IMP1) inhibits production of infectious HIV-1 particles through adversely affecting virus maturation. Here, we report that IMP1 interacts with HIV-1 Rev protein and its ectopic expression causes relocation of Rev from the nucleus to the cytoplasm. In accordance with this observation, ectopic expression of IMP1 severely diminishes Rev-dependent expression of CAT enzyme and disturbs HIV-1 RNA expression by causing accumulation of the multiple spliced viral RNA. Results of mutagenesis analysis further reveal that the KH4 domain represents the key element of IMP1 in modulating HIV-1 RNA expression. Taken together, these data suggest, in addition to hampering virus assembly, that IMP1 also has an effect on Rev-dependent viral RNA expression.

p53-dependent translational control of senescence and transformation via 4E-BPs.

eIF4E, the mRNA 5' cap-binding translation initiation factor, is overexpressed in numerous cancers and is implicated in mechanisms underlying oncogenesis and senescence. 4E-BPs (eIF4E-binding proteins) inhibit eIF4E activity, and thereby act as suppressors of eIF4E-dependent pathways. Here, we show that tumorigenesis is increased in p53 knockout mice that lack 4E-BP1 and 4E-BP2. However, primary fibroblasts lacking 4E-BPs, but expressing p53, undergo premature senescence and resist oncogene-driven transformation. Thus, the p53 status governs 4E-BP-dependent senescence and transformation. Intriguingly, the 4E-BPs engage in senescence via translational control of the p53-stabilizing protein, Gas2. Our data demonstrate a role for 4E-BPs in senescence and tumorigenesis and highlight a p53-mediated mechanism of senescence through a 4E-BP-dependent pathway.

The packaging of human immunodeficiency virus type 1 RNA is restricted by overexpression of an RNA helicase DHX30.

RNA helicases are a large family of proteins that are able to unwind RNA duplex and remodel the structure of RNA-protein (RNP) complexes using energy derived from hydrolysis of nucleotide triphosphates (NTPs). Every step of cellular RNA metabolism involves the activity of RNA helicases. Not surprisingly, more and more RNA helicases are reported to participate in the replication of viruses including the human immunodeficiency virus type 1 (HIV-1). Here, we provide evidence that overexpression of an RNA helicase named DHX30 enhances HIV-1 gene expression, but leads to the generation of viruses that package significantly low levels of viral RNA and exhibit severely decreased infectivity. These data reveal the complex roles of DHX30 in HIV-1 replication and implicate an inhibitory activity of DHX30 in HIV-1 RNA packaging.

The requirement of the DEAD-box protein DDX24 for the packaging of human immunodeficiency virus type 1 RNA.

RNA helicases play important roles in RNA metabolism. Human immunodeficiency virus type 1 (HIV-1) does not carry its own RNA helicase, the virus thus needs to exploit cellular RNA helicases to promote the replication of its RNA at various steps such as transcription, folding and transport. In this study, we report that knockdown of a DEAD-box protein named DDX24 inhibits the packaging of HIV-1 RNA and thus diminishes viral infectivity. The decreased viral RNA packaging as a result of DDX24-knockdown is observed only in the context of the Rev/RRE (Rev response element)-dependent but not the CTE (constitutive transport element)-mediated nuclear export of viral RNA, which is explained by the specific interaction of DDX24 with the Rev protein. We propose that DDX24 acts at the early phase of HIV-1 RNA metabolism prior to nuclear export and the consequence of this action extends to the viral RNA packaging stage during virus assembly.