Cryo-EM structures of perforin-2 in isolation and assembled on a membrane suggest a mechanism for pore formation.

Perforin-2 (PFN2, MPEG1) is a key pore-forming protein in mammalian innate immunity restricting intracellular bacteria proliferation. It forms a membrane-bound pre-pore complex that converts to a pore-forming structure upon acidification; but its mechanism of conformational transition has been debated. Here we used cryo-electron microscopy, tomography and subtomogram averaging to determine structures of PFN2 in pre-pore and pore conformations in isolation and bound to liposomes. In isolation and upon acidification, the pre-assembled complete pre-pore rings convert to pores in both flat ring and twisted conformations. On membranes, in situ assembled PFN2 pre-pores display various degrees of completeness; whereas PFN2 pores are mainly incomplete arc structures that follow the same subunit packing arrangements as found in isolation. Both assemblies on membranes use their P2 ß-hairpin for binding to the lipid membrane surface. Overall, these structural snapshots suggest a molecular mechanism for PFN2 pre-pore to pore transition on a targeted membrane, potentially using the twisted pore as an intermediate or alternative state to the flat conformation, with the capacity to cause bilayer distortion during membrane insertion.

Perforin-2 clockwise hand-over-hand pre-pore to pore transition mechanism.

Perforin-2 (PFN2, MPEG1) is a pore-forming protein that acts as a first line of defense in the mammalian immune system, rapidly killing engulfed microbes within the phagolysosome in macrophages. PFN2 self-assembles into hexadecameric pre-pore rings that transition upon acidification into pores damaging target cell membranes. Here, using high-speed atomic force microscopy (HS-AFM) imaging and line-scanning and molecular dynamics simulation, we elucidate PFN2 pre-pore to pore transition pathways and dynamics. Upon acidification, the pre-pore rings (pre-pore-I) display frequent, 1.8 s-1, ring-opening dynamics that eventually, 0.2 s-1, initiate transition into an intermediate, short-lived, ~75 ms, pre-pore-II state, inducing a clockwise pre-pore-I to pre-pore-II propagation. Concomitantly, the first pre-pore-II subunit, undergoes a major conformational change to the pore state that propagates also clockwise at a rate ~15 s-1. Thus, the pre-pore to pore transition is a clockwise hand-over-hand mechanism that is accomplished within ~1.3 s. Our findings suggest a clockwise mechanism of membrane insertion that with variations may be general for the MACPF/CDC superfamily.

Structure and assembly of cargo Rubisco in two native α-carboxysomes.

Carboxysomes are a family of bacterial microcompartments in cyanobacteria and chemoautotrophs. They encapsulate Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and carbonic anhydrase catalyzing carbon fixation inside a proteinaceous shell. How Rubisco complexes pack within the carboxysomes is unknown. Using cryo-electron tomography, we determine the distinct 3D organization of Rubisco inside two distant α-carboxysomes from a marine α-cyanobacterium Cyanobium sp. PCC 7001 where Rubiscos are organized in three concentric layers, and from a chemoautotrophic bacterium Halothiobacillus neapolitanus where they form intertwining spirals. We further resolve the structures of native Rubisco as well as its higher-order assembly at near-atomic resolutions by subtomogram averaging. The structures surprisingly reveal that the authentic intrinsically disordered linker protein CsoS2 interacts with Rubiscos in native carboxysomes but functions distinctively in the two α-carboxysomes. In contrast to the uniform Rubisco-CsoS2 association in the Cyanobium α-carboxysome, CsoS2 binds only to the Rubiscos close to the shell in the Halo α-carboxysome. Our findings provide critical knowledge of the assembly principles of α-carboxysomes, which may aid in the rational design and repurposing of carboxysome structures for new functions.

Structural, Functional and Computational Studies of Membrane Recognition by Plasmodium Perforin-Like Proteins 1 and 2.

Perforin-like proteins (PLPs) play key roles in mechanisms associated with parasitic disease caused by the apicomplexan parasites Plasmodium and Toxoplasma. The T. gondii PLP1 (TgPLP1) mediates tachyzoite egress from cells, while the five Plasmodium PLPs carry out various roles in the life cycle of the parasite and with respect to the molecular basis of disease. Here we focus on Plasmodium vivax PLP1 and PLP2 (PvPLP1 and PvPLP2) compared to TgPLP1. Determination of the crystal structure of the membrane-binding APCß domain of PvPLP1 reveals notable differences with TgPLP1, reflected in its inability to bind lipid bilayers as TgPLP1 and PvPLP2 do. Molecular dynamics simulations combined with site-directed mutagenesis and functional assays allow dissection of the binding interactions of TgPLP1 and PvPLP2 on lipid bilayers, and reveal similar tropisms for lipids enriched in the inner leaflet of the mammalian plasma membrane. In addition PvPLP2 displays a secondary synergistic interaction side-on from its principal bilayer interface. This study underlines the substantial differences between the biophysical properties of the APCß domains of apicomplexan PLPs, which reflect their significant sequence diversity. Such differences will be important factors in determining the cell targeting and membrane-binding activity of the different proteins in parasitic life cycles and disease.

The Arabidopsis STE20/Hippo kinase SIK1 regulates polarity independently of PIN proteins.

Polarity is a feature of life. In higher plants, non-autonomous polarity is largely directed by auxin, the morphogen that drives its own polarized flow, Polar Auxin Transport (PAT), to guide patterning events such as phyllotaxis and tropism. The plasma membrane-localized PIN-FORMED (PIN) auxin efflux carriers are rate-limiting factors in PAT. In yeasts and metazoans, the STE20 kinases are key players in cell polarity. We had previously characterized SIK1 as a STE20/Hippo orthologue in Arabidopsis and confirmed its function in mitotic exit and organ growth. Here we explore the possible link between SIK1, auxin, PIN, and polarity. Abnormal phyllotaxis and gravitropism were observed in sik1. sik1 was more sensitive to exogenous auxin in primary root elongation and lateral root emergence. RNA-Seq revealed reduced expression in auxin biosynthesis genes and induced expression of auxin flux carriers in sik1. However, normal tissue- and sub-cellular localization patterns of PIN1 and PIN2 were observed in sik1. The dark-induced vacuolar degradation of PIN2 also appeared normal in sik1. An additive phenotype was observed in the sik1 pin1 double mutant, indicating that SIK1 does not directly regulate PIN1. The polarity defects of sik1 are hence unlikely mediated by PINs and await future exploration.

Live Imaging of Arabidopsis Leaf and Vegetative Meristem Development.

Plants develop lateral organs such as leaves and flowers throughout their post-embryonic life from a structure called the shoot apical meristem (SAM), located at the plant shoot apex. This process is highly dynamic, and therefore in order to understand meristem and organ development, it is critical to be able to analyze these processes with high temporal and spatial resolution. Although several protocols have been published for imaging the Arabidopsis inflorescence meristem, gaining access to the vegetative meristem for imaging has been considered more difficult. Here we describe a method to dissect young Arabidopsis seedlings in order to obtain a clear view of the vegetative meristem and young leaf primordia using confocal microscopy.

An integrated analysis of cell-type specific gene expression reveals genes regulated by REVOLUTA and KANADI1 in the Arabidopsis shoot apical meristem.

In the Arabidopsis thaliana shoot apical meristem (SAM) the expression domains of Class III Homeodomain Leucine Zipper (HD-ZIPIII) and KANADI (KAN) genes are separated by a narrow boundary region from which new organs are initiated. Disruption of this boundary through either loss of function or ectopic expression of HD-ZIPIII and KAN causes ectopic or suppression of organ formation respectively, raising the question of how these transcription factors regulate organogenesis at a molecular level. In this study we develop a multi-channel FACS/RNA-seq approach to characterize global patterns of gene expression across the HD-ZIPIII-KAN1 SAM boundary. We then combine FACS, RNA-seq and perturbations of HD-ZIPIII and KAN expression to identify genes that are both responsive to REV and KAN1 and normally expressed in patterns that correlate with REV and KAN1. Our data reveal that a significant number of genes responsive to REV are regulated in opposite ways depending on time after induction, with genes associated with auxin response and synthesis upregulated initially, but later repressed. We also characterize the cell type specific expression patterns of auxin responsive genes and identify a set of genes involved in organogenesis repressed by both REV and KAN1.

Structure and mechanism of bactericidal mammalian perforin-2, an ancient agent of innate immunity.

Perforin-2 (MPEG1) is thought to enable the killing of invading microbes engulfed by macrophages and other phagocytes, forming pores in their membranes. Loss of perforin-2 renders individual phagocytes and whole organisms significantly more susceptible to bacterial pathogens. Here, we reveal the mechanism of perforin-2 activation and activity using atomic structures of pre-pore and pore assemblies, high-speed atomic force microscopy, and functional assays. Perforin-2 forms a pre-pore assembly in which its pore-forming domain points in the opposite direction to its membrane-targeting domain. Acidification then triggers pore formation, via a 180° conformational change. This novel and unexpected mechanism prevents premature bactericidal attack and may have played a key role in the evolution of all perforin family proteins.

Arbutin protects HK-2 cells against high glucose-induced apoptosis and autophagy by up-regulating microRNA-27a.

Arbutin (ARB) has been widely used in skin pigmentation disorders. Nevertheless, the involvements of ARB in diabetic nephropathy (DN) are still unknown. We investigated the functions of ARB in high glucose (HG)-induced cell apoptosis and autophagy in HK-2 cells. Cell viability was examined through CCK-8 in HK-2 cells after disposal with 45 mM glucose and ARB (10-50 µM). Flow cytometry and western blot tested cell apoptosis and the related protein levels in HK-2 cells after 45 mM glucose and 50 µM ARB administration. RT-qPCR delved microRNA (miR)-27a expression in HG and ARB co-treated HK-2 cells. Effect of miR-27a on ARB affected cell apoptosis and autophagy was investigated after miR-27a inhibitor transfection. JNK and mTOR pathways were finally assessed by western blot. ARB alleviated HG-induced cell apoptosis, autophagy and regulated the related protein levels in HK-2 cells. MiR-27a expression was reduced in HG-treated cells, but was accelerated in HG and ARB co-treated HK-2 cells with the increased concentration. Inhibition of miR-27a apparently abolished the outcomes of ARB in HG-induced HK-2 cells apoptosis and autophagy. Besides, ARB blocked JNK and mTOR pathways by regulating miR-27a. The findings demonstrated that ARB alleviated apoptosis and autophagy in HG-treated HK-2 cells by regulating miR-27a/JNK/mTOR axis.

Silence of lncRNA GAS5 alleviates high glucose toxicity to human renal tubular epithelial HK-2 cells through regulation of miR-27a.

Renal tubular damage caused by persistent high glucose environment has been found to contribute to diabetic nephropathy. This study explored the effects of lncRNA growth arrest-specific 5 (GAS5) on high glucose-stimulated human renal tubular epithelial HK-2 damage, as well as the possible internal molecular mechanism. Viability and apoptosis of HK-2 cells were assessed with the help of CCK-8 assay and Annexin V-FITC/PI staining, respectively. Cell transfection was used to change the expression of GAS5, miR-27a and BNIP3. We found that high glucose stimulation suppressed HK-2 cell viability but induced cell apoptosis. The expression of GAS5 was increased in HK-2 cells under high glucose environment. Silence of GAS5 mitigated the high glucose-caused HK-2 cell viability reduction and apoptosis. Overexpression of miR-27a reversed the effects of GAS5 on high glucose-stimulated HK-2 cells. Overexpression of BNIP3 aggravated the high glucose-caused HK-2 cell viability reduction, apoptosis and activation of JNK pathway. Knockdown of BNIP3 had opposite effects. In conclusion, this research further confirmed the pro-apoptotic roles of GAS5 in renal tubular epithelial cells under high glucose environment. Silence of GAS5 alleviated high glucose toxicity to human renal tubular epithelial HK-2 cells might be via down-regulating miR-27a and BNIP3, and then inactivating JNK pathway. Highlights HG suppresses HK-2 cell viability, but promotes cell apoptosis; HG enhances the expression of GAS5 in HK-2 cells; Silence of GAS5 alleviates the HG-caused HK-2 cell toxicity; miR-27a participates in the effects of GAS5 silencing on HG-stimulated HK-2 cells; BNIP3 is regulated by miR-27a and related to the HG toxicity to HK-2 cells.

Cell type boundaries organize plant development.

In plants the dorsoventral boundary of leaves defines an axis of symmetry through the centre of the organ separating the top (dorsal) and bottom (ventral) tissues. Although the positioning of this boundary is critical for leaf morphogenesis, how the boundary is established and how it influences development remains unclear. Using live-imaging and perturbation experiments we show that leaf orientation, morphology and position are pre-patterned by HD-ZIPIII and KAN gene expression in the shoot, leading to a model in which dorsoventral genes coordinate to regulate plant development by localizing auxin response between their expression domains. However we also find that auxin levels feedback on dorsoventral patterning by spatially organizing HD-ZIPIII and KAN expression in the shoot periphery. By demonstrating that the regulation of these genes by auxin also governs their response to wounds, our results also provide a parsimonious explanation for the influence of wounds on leaf dorsoventrality.

The Hippo/STE20 homolog SIK1 interacts with MOB1 to regulate cell proliferation and cell expansion in Arabidopsis.

Multicellular organisms co-ordinate cell proliferation and cell expansion to maintain organ growth. In animals, the Hippo tumor suppressor pathway is a master regulator of organ size. Central to this pathway is a kinase cascade composed of Hippo and Warts, and their activating partners Salvador and Mob1/Mats. In plants, the Mob1/Mats homolog MOB1A has been characterized as a regulator of cell proliferation and sporogenesis. Nonetheless, no Hippo homologs have been identified. Here we show that the Arabidopsis serine/threonine kinase 1 (SIK1) is a Hippo homolog, and that it interacts with MOB1A to control organ size. SIK1 complements the function of yeast Ste20 in bud site selection and mitotic exit. The sik1 null mutant is dwarf with reduced cell numbers, endoreduplication, and cell expansion. A yeast two-hybrid screen identified Mob1/Mats homologs MOB1A and MOB1B as SIK1-interacting partners. The interaction between SIK1 and MOB1 was found to be mediated by an N-terminal domain of SIK1 and was further confirmed by bimolecular fluorescence complementation. Interestingly, sik1 mob1a is arrested at the seedling stage, and overexpression of neither SIK1 in mob1a nor MOB1A in sik1 can rescue the dwarf phenotypes, suggesting that SIK1 and MOB1 may be components of a larger protein complex. Our results pave the way for constructing a complete Hippo pathway that controls organ growth in higher plants.