The Conserved Colletotrichum spp. Effector Candidate CEC3 Induces Nuclear Expansion and Cell Death in Plants

Plant pathogens secrete proteins, known as effectors, that promote infection by manipulating host cells. Members of the phytopathogenic fungal genus Colletotrichum collectively have a broad host range and generally adopt a hemibiotrophic lifestyle that includes an initial biotrophic phase and a later necrotrophic phase. We hypothesized that Colletotrichum fungi use a set of conserved effectors during infection to support the two phases of their hemibiotrophic lifestyle. This study aimed to examine this hypothesis by identifying and characterizing conserved effectors among Colletotrichum fungi. Comparative genomic analyses using genomes of ascomycete fungi with different lifestyles identified seven effector candidates that are conserved across the genus Colletotrichum. Transient expression assays showed that one of these putative conserved effectors, CEC3, induces nuclear expansion and cell death in Nicotiana benthamiana, suggesting that CEC3 is involved in promoting host cell death during infection. Nuclear expansion and cell death induction were commonly observed in CEC3 homologs from four different Colletotrichum species that vary in host specificity. Thus, CEC3 proteins could represent a novel class of core effectors with functional conservation in the genus Colletotrichum.

Strategic maneuvering in extended polylogues

This paper presents the analysis of the Hungarian nuclear expansion controversy using a conceptual framework that links strategic maneuvering with an extended polylogical controversy and evaluates the strategic maneuvering of political, environmentalist and expert actors. The paper aims to show that the three aspects of strategic maneuvering (audience demand, topical potential, presentational devices) are flexible enough that they can be analyzed when the object of study is not a spatially and temporally localized argumentative situation, but a decade-long debate with multiple actors. In 2014, Hungary signed a deal with Russia to finance 80% of the investment costs and supply two new reactors to maintain the 40–50% of nuclear energy in the national energy production.

The Conserved Colletotrichum spp. Effector CEC3 Induces Nuclear Expansion and Cell Death in Plants

Plant pathogens secrete small proteins, known as effectors, that promote infection by manipulating host cells. Members of the phytopathogenic fungal genus Colletotrichum collectively have a broad host range and generally adopt a hemibiotrophic lifestyle that includes an initial biotrophic phase and a later necrotrophic phase. We hypothesized that Colletotrichum fungi use a set of conserved effectors during infection to support the two phases of their hemibiotrophic lifestyle. This study aimed to examine this hypothesis by identifying and characterizing conserved effectors among Colletotrichum fungi. Comparative genomic analyses using genomes of ascomycete fungi with different lifestyles identified seven effector candidates that are conserved across the genus Colletotrichum. Transient expression assays showed that one of these conserved effectors, CEC3, induces nuclear expansion and cell death in Nicotiana benthamiana, suggesting that CEC3 is involved in promoting host cell death during infection. Nuclear expansion and cell death induction were commonly observed in CEC3 homologs from four different Colletotrichum species that vary in host specificity. Thus, CEC3 proteins could represent a novel class of core effectors with functional conservation in the genus Colletotrichum.

Cardiolipin targets a dynamin related protein to the nuclear membrane

Dynamins are targeted to specific cellular membranes that they remodel via membrane fusion or fission. The molecular basis of conferring specificity to dynamins for their target membrane selection is not known. Here, we report a mechanism of nuclear membrane recruitment of Drp6, a dynamin member in Tetrahymena thermophila. Recruitment of Drp6 depends on a domain that binds to cardiolipin-rich bilayers. Consistent with this, nuclear localization of Drp6 was inhibited either by depleting cellular cardiolipin (CL) or by substituting a single amino acid residue that abolished Drp6 interactions with CL. Inhibition of CL synthesis, or perturbation in Drp6 recruitment to nuclear membrane, caused defects in the formation of new macronuclei post-conjugation. Taken together, our results elucidate a molecular basis of target membrane selection by a nuclear dynamin, and establish the importance of a defined membrane-binding domain and its target lipid in facilitating nuclear expansion.

Yolk platelets impede nuclear expansion in Xenopus embryos

ABSTRACTDuring metazoan early embryogenesis, the intracellular properties of proteins and organelles change dynamically through rapid cleavage. In particular, a change in the nucleus size is known to contribute to embryonic development-dependent cell cycle and gene expression regulation. Here, we compared the nuclear sizes of various blastomeres from developing Xenopus embryos and analyzed the mechanisms that control the nuclear expansion dynamics by manipulating the amount of intracellular components in a cell-free system. There was slower nuclear expansion during longer interphase durations in blastomeres from vegetal hemispheres than those from animal hemispheres. Furthermore, upon recapitulating interphase events by manipulating the concentration of yolk platelets, which are originally rich in the vegetal blastomeres, in cell-free cytoplasmic extracts, there was slower nuclear expansion and DNA replication as compared to normal yolk-free conditions. Under these conditions, the supplemented yolk platelets accumulated around the nucleus in a microtubule-dependent manner and impeded organization of the endoplasmic reticulum network. Overall, we propose that yolk platelets around the nucleus reduce membrane supply from the endoplasmic reticulum to the nucleus, resulting in slower nuclear expansion in the yolk-rich vegetal blastomeres.

A light way for nuclear cell biologists

The nucleus is a very complex organelle present in eukaryotic cells. Having the crucial task to safeguard, organize and manage the genetic information, it must tightly control its molecular constituents, its shape and its internal architecture at any given time. Despite our vast knowledge of nuclear cell biology, much is yet to be unraveled. For instance, only recently we came to appreciate the existence of a dynamic nuclear cytoskeleton made of actin filaments that regulates processes such as gene expression, DNA repair and nuclear expansion. This suggests further exciting discoveries ahead of us. Modern cell biologists embrace a new methodology relying on precise perturbations of cellular processes that require a reversible, highly spatially-confinable, rapid, inexpensive and tunable external stimulus: light. In this review, we discuss how optogenetics, the state-of-the-art technology that uses genetically-encoded light-sensitive proteins to steer biological processes, can be adopted to specifically investigate nuclear cell biology.

DNA content contributes to nuclear size control in Xenopus laevis

Physical properties of DNA, including quantity and chromatin structure within the nucleus, contribute to nuclear expansion dynamics and final nuclear size in Xenopus egg extracts and embryos.

Cardiolipin targets a dynamin related protein to the nuclear membrane

ABSTRACTDynamins are large cytoplasmic GTPases that are targeted to specific cellular membranes which they remodel via membrane fusion or fission. Although the mechanism of target membrane selection by dynamins has been studied, the molecular basis of conferring specificity to bind specific lipids on the target membranes is not known in any of the family members. Here, we report a mechanism of nuclear membrane recruitment of Drp6 that is involved in nuclear remodeling in Tetrahymena thermophila. Recruitment of Drp6 depends on a domain that binds to cardiolipin-rich bilayers. Consistent with this, the nuclear localization of wildtype Drp6 was inhibited by depleting cardiolipin in the cell. Cardiolipin binding was blocked with a single amino acid substitution (I553M) in the membrane-binding domain of Drp6. Importantly, the I553M substitution was sufficient to block nuclear localization without affecting other properties of Drp6. Consistent with this result, co-expression of wildtype Drp6 was sufficient to rescue the localization defect of I553M variant in Tetrahymena. Inhibition of cardiolipin synthesis or perturbation in Drp6 recruitment to nuclear membrane caused defects in the formation of new macronuclei post-conjugation. Taken together, our results elucidate a molecular basis of target membrane selection by a nuclear dynamin, and establish the importance of a defined membrane-binding domain and its target lipid in facilitating nuclear expansion.

Post-mitotic expansion of cell nuclei requires ACTN4-mediated nuclear actin filament bundling

The actin cytoskeleton operates in a multitude of cellular processes including cell shape and migration, mechanoregulation, as well as membrane or organelle dynamics. However, its filamentous properties and functions inside the mammalian cell nucleus are less well explored. We previously described transient actin assembly at mitotic exit that promotes nuclear expansion during chromatin decondensation. Here, we identify non-muscle ACTN4 as a critical regulator to facilitate F-actin formation, reorganization and bundling during postmitotic nuclear expansion. ACTN4 binds to nuclear actin filaments and ACTN4 clusters associate with nuclear F-actin in a highly dynamic fashion. ACTN4 but not ACTN1 is required for proper postmitotic nuclear volume expansion, mediated by its actin binding domain. Using super-resolution imaging to quantify actin filament numbers and widths in individual nuclei we find that ACTN4 is necessary for postmitotic nuclear actin assembly and actin filament bundling. Our findings uncover a nuclear cytoskeletal function for ACTN4 to control nuclear size during mitotic cell division.