Polycomb Group蛋白复合体与植物春化作用

Polycomb Group(PcG)蛋白能形成Polycomb Repressive Complex 1(PRC1)和PRC2等复合体,通过一个保守且表观遗传的机制调节基因表达并控制动植物的发育。拟南芥中由VERNALIZATION2参与形成的PRC2复合体(VRN2-PRC2)在春化过程中能对主要开花抑制基因FLOWER LOCUS C(FLC)的染色质进行组蛋白甲基化修饰,形成H3K27me3(组蛋白H3第27位赖氨酸三甲基化)等转录抑制标记,从而抑制FLC转录,促进开花。虽然麦类作物的春化机理与拟南芥有较大差异,但最近的研究表明麦类作物春化过程也受PcG蛋白调控。文章对拟南芥PcG蛋白介导的春化调节机制进行综述,期望能对植物尤其是麦类作物的春化机理研究提供资料。     

水稻SDG736抑制拟南芥FLC表达调控开花时间

130～150个氨基酸组成SET (Su (var) 3-9, Enhancer-of-zeste, Trithorax)结构域构成了组蛋白赖氨酸甲基转移酶特异性催化位点。SET结构域蛋白在进化上高度保守,广泛调控植物的生长发育。进化分析结果显示水稻SET结构域家族成员可分为7个不同的亚家族(KMT1, KMT2, KMT3, KMT6, KMT7, S-ET和RMT)。KMT3亚家族可能涉及开花调控或花的发育,其中包含5个拟南芥基因和5个水稻同源基因。拟南芥SDG4通过H3K4/K36甲基化的活性调控花发育,结果表明水稻同源基因SDG736超量表达,可促进拟南芥开花。对拟南芥开花途径相关的基因进行定量分析显示,超量表达的SDG736拟南芥植株中FLC基因表达量降低,而SCO1基因的表达量增加。     

植物组蛋白赖氨酸化修饰参与基因表达调控的机理

组蛋白共价修饰作为表观遗传修饰的重要部分,主要包括乙酰化和甲酰化、甲基化、磷酸化、泛素化和SUMO化等,它们形成一个复杂的网络共同调控基因的表达,其中组蛋白甲基化修饰成为研究的热点,甲基化主要发生在赖氨酸残基上。近年来,随着有关植物组蛋白赖氨酸甲基化修饰研究的不断深入,发现其通过改变自身赖氨酸残基的甲基化状态和甲基化程度,形成转录激活或者转录抑制标记,调控基因的表达,在植物开花和逆境胁迫的响应过程中起着至关重要的作用。H3组蛋白的赖氨酸甲基化修饰能够调控FLC基因和有关抗性基因的表达,具体表现为:H3K4的三甲基化促进FLC的表达,H3K27的三甲基化则抑制FLC的表达;H3K4me3作为转录激活标记,可激活PtdIns5P基因的表达,启动响应干旱的脂质合成信号通路,响应干旱胁迫;相反,H3K27me3作为一种转录抑制标记,低水平的H3K27me3诱导COR15A和ATGOLS3基因表达,它们分别编码叶绿体低温保护蛋白Cor15am和肌醇半乳糖合成酶GOLS,以抵抗寒冷胁迫。文章主要综述了植物组蛋白赖氨酸甲基化修饰参与DNA甲基化、开花过程以及应答逆境胁迫的分子机制。     

植物PHD结构域蛋白的结构与功能特性

植物同源结构域(plant homeodomain,PHD)是锌指结构域家族的一类转录调控因子,其最主要的功能是可以识别各种组蛋白修饰密码,包括组蛋白甲基化和乙酰化等;此外PHD结构域还可以与DNA结合。含有PHD结构域的蛋白,或者本身具有组蛋白修饰酶活性,或者可以与各类组蛋白修饰酶相互作用,还有部分与DNA甲基化相关,具有E3泛素连接酶活性,或者还可以作为染色质重塑因子,以各种不同的作用方式,在植物的生长发育过程中发挥了重要的作用。本文主要综述了结合各种类型组蛋白(包括H3K4me3/0、H3K9me3、H3R2和H3K14ac)以及DNA的PHD结构域的结构特点及其结合特异性、PHD结构域在植物中的进化保守性以及植物中已经发现的含有PHD结构域蛋白的功能及作用机制,为进一步了解该类蛋白在植物生长发育过程中如何发挥作用提供了参考。     

染色质乙酰化相关BRD蛋白家族

Bromodomain结构域首先在果蝇蛋白质Brahma中发现,折叠模式独特且高度保守,是最早也是截至目前公认唯一可与乙酰化赖氨酸结合的结构域。BRD蛋白通过结合不同的蛋白质或者定位蛋白质到细胞核发挥精细调节作用。BRD蛋白复合物常特异性识别并结合到染色质组蛋白H3/H4特定的乙酰化赖氨酸残基,从而影响靶基因的转录翻译;该蛋白复合物功能异常通常与多种疾病的发生相关联,表明对转录翻译调节有重要意义。但迄今为止,BRD蛋白复合物修饰染色质机理不明,现有研究提示BRD蛋白复合物维持染色质乙酰化状态,也可以与染色质组蛋白其它位点结合,从整体水平增强组蛋白乙酰化精度和效率。     

春化作用相关基因FLC的研究进展

拟南芥春化作用相关基因FLOWERING LOCUS C（FLC）属于MADS盒基因,它编码的蛋白转录因子对开花具抑制作用。春化作用通过负调控FLC的转录及蛋白表达水平,促进拟南芥的某些晚花生态型和晚花突变体开花。主要介绍了FLC基因在春化途径中的关键作用,及其春化作用通过FLC基因与其它开花途径相联系等内容。     

植物同源结构域(PHD结构域)——组蛋白密码的解读器

植物同源结构域(plant homeodomain,PHD结构域),是真核生物中一种进化保守的锌指结构域.多种调控基因转录、细胞周期、凋亡的蛋白质含有PHD结构域.研究表明,PHD结构域涉及多种功能,包括蛋白质相互作用,特别是同核小体组蛋白的作用.目前认为,各种组蛋白修饰(包括甲基化、乙酰化、磷酸化、泛素化等)的模式和组合,调节染色质状态和基因转录活性,并提出了组蛋白密码理论.PHD指结构域能特异性识别组蛋白的甲基化(修饰)密码,可能是组蛋白密码的一种重要解读器.     

MADS-box基因控制植物成花的分子机理

植物花器官的发育和开花是植物生殖发育中最重要的过程,植物在长期的进化过程中产生了春化(低温)途径、自主途径、光周期途径以及不依赖于光温环境条件的赤霉素信号途径来适应多变的环境和调控植物开花过程。本文综述了模式植物拟南芥中由LEAFY(LFY)、CONSTANS(CO)、FLOWERING LOCUSC(FLC)、FLOW ERING LOCUS T(FT)和SUPPRESSOR OF OVEREXPRESSION OF CO1(SOC1)等基因构成的双子叶植物响应光温条件变化的开花调控网络;以及大麦、小麦中由VERNALIZATION1(VRN1)、VRN2、ODD-SOC2(OS2)和拟南芥CO、FT同源基因构成的禾本科植物开花调控网络。其中最重要的是转录调控因子MADS-box基因FLC、SOC1、VRN1和OS2,并发现组蛋白的乙酰化/脱乙酰化,赖氨酸的甲基化/脱甲基化在调控FLC、VRN1染色质活性状态及基因表达,从而产生开花控制的机理。这些研究发现将有助于对具有重要经济价值的单双子叶植物,通过生物技术手段改良其品种特性以应对非生物逆境,特别是低温胁迫的指导。     

伴胞细胞中表达的组蛋白脱甲基化酶感受发育信号调控开花时间

开花是高等植物发育过程中一个非常重要的转化过程,它能够保证植物的正常发育和后代的延续,并且有重要的农业价值和观赏价值[1]．开花时间的调控是一个非常复杂的过程,受到自身发育信号和外部环境因素的共同影响[2-3]．FLC是拟南芥开花调节过程中的中心抑制因子,其在拟南芥顶端分生组织和叶片维管束的伴胞细胞中均有表达,并且这两个部位的FLC对开花时间都有重要的调节作用[4]．目前已知的多数影响开花的通路都通过调节顶端FLC的表达来调控植物开花时间,关于伴胞细胞中的FLC如何被调控的研究还非常少[1, 3]． 在动植物中都存在一类具有JmjC结构域的蛋白质,是一类保守的组蛋白脱甲基化酶[5]．我们实验室最近的工作表明,JMJ18是一个受植物自身发育调节的H3K4脱甲基化酶,JMJ18主要在伴胞细胞中表达,通过特异调节伴胞细胞中的FLC调控植物开花时间[6]． Yang等[6]实验证实在体外全长的JMJ18可以特异性地以H3K4m3的多肽为底物,脱掉其上一个甲基生成H3K4m2．在拟南芥中,JMJ18主要在伴胞细胞中表达,并且表达水平受到植物自身发育进程的调控[4]．JMJ18功能缺失突变体呈现弱的晚花表型,而JMJ18的超表达植株呈现明显的早花表型,说明JMJ18参与了拟南芥开花时间的调控[4].尽管多个具有JmjC结构域的组蛋白脱甲基化酶,如 JMJ14、ELF6/JMJ11、REF6/JMJ12等都参与了拟南芥开花时间的调节,但是机制都不太清楚[5, 7],并且目前没有发现可以直接调控FLC的JmjC蛋白．Yang等的实验证实JMJ18可以结合到FLC的染色质上,通过降低FLC的染色质H3K4m3和H3K4m2修饰抑制FLC表达．FLC表达水平的降低导致FT表达的释放,促进FT在伴胞细胞中积累．积累的FT从伴胞细胞进入筛管组织,进而运输到顶端分生组织,与顶端分生组织特异性表达的bZIP转录因子FD直接相互作用,通过调节下游基因SOC1和AP1调控植物开花进程(图1)． 最近的研究发现,植物开花时间除了受到春化作用、自主途径、光周期途径、GA途径等调控以外,还可以通过自身年龄衡量因子miR156和其靶基因SQUAMOSA PROMOTER BINDING-LIKE (SPLs)调节开花进程[8]．Yang等实验证实：JMJ18主要在韧皮部的伴胞细胞表达．并且同miR156类似,在植物营养生长时期,JMJ18随着发育进程的深入表达水平逐渐升高．SUC2启动子驱动JMJ18在维管伴胞细胞中表达时也出现早花表型并且依赖于FT．这些研究结果表明,同miR156类似,JMJ18受植物自身发育调节,也可能作为自身年龄衡量因子调控植物开花时间,不同点是JMJ18是通过组蛋白修饰直接调节FLC表达调控开花时间的自身年龄衡量因子．即可能有两条感受自身年龄的途径：miR156-SPLs和JMJ18-FLC/MAFs途径,让人感兴趣的是两个因子都是表观遗传调控因子,而且在每个途径中均是前者负调控后者,而且后者均为一个转录因子基因家族,这两个途径最后都调控FT表达．这两个途径之间的关系也是一个有待于研究的科学问题,这可能会对于我们理解自身年龄衡量因子在植物开花进程中的作用有一定的启示．     

DOT1—— 一类新的组蛋白赖氨酸甲基转移酶

组蛋白甲基化是一种重要的组蛋白共价修饰, 在染色质结构和基因表达的调控过程中起着重要的、多样化的作用。DOT1催化核心球体部位的组蛋白H3第79位赖氨酸(H3K79)使其发生甲基化, 是首个被发现的无SET结构域的组蛋白赖氨酸甲基转移酶, 代表了一类新的组蛋白赖氨酸甲基转移酶。DOT1及H3K79甲基化的特点决定了其可能具有重要的、特殊的生物学功能。文章重点综述了DOT1蛋白的结构及特点, DOT1及H3K79甲基化的生物学功能以及组蛋白泛素化修饰对H3K79甲基化的反式调控。     

Polycomb proteins regulate the quantitative induction of VERNALIZATION INSENSITIVE 3 in response to low temperatures

Vernalization, the promotion of flowering in response to low temperatures, is one of the best characterized examples of epigenetic regulation in plants. The promotion of flowering is proportional to the duration of the cold period, but the mechanism by which plants measure time at low temperatures has been a long‐standing mystery. We show that the quantitative induction of the first gene in the Arabidopsis vernalization pathway, VERNALIZATION INSENSITIVE 3 (VIN3), is regulated by the components of Polycomb Response Complex 2, which trimethylates histone H3 lysine 27 (H3K27me3). In differentiated animal cells, H3K27me3 is mostly associated with long‐term gene repression, whereas, in pluripotent embyonic stem cells, many cell lineage‐specific genes are inactive but exist in bivalent chromatin that carries both active (H3K4me3) and repressive (H3K27me3) marks on the same molecule. During differentiation, bivalent domains are generally resolved to an active or silent state. We found that H3K27me3 maintains VIN3 in a repressed state prior to cold exposure; this mark is not removed during VIN3 induction. Instead, active VIN3 is associated with bivalently marked chromatin. The continued presence of H3K27me3 ensures that induction of VIN3 is proportional to the duration of the cold, and that plants require prolonged cold to promote the transition to flowering. The observation that Polycomb proteins control VIN3 activity defines a new role for Polycomb proteins in regulating the rate of gene induction.     

Coordination of the Vernalization Response through a VIN3 and FLC Gene Family Regulatory Network in Arabidopsis

Vernalization is an environmentally induced epigenetic switch in which winter cold triggers epigenetic silencing of floral repressors and thus provides competence to flower in spring. Vernalization triggers the recruitment of chromatin-modifying complexes to a clade of flowering repressors that are epigenetically silenced via chromatin modifications. In Arabidopsis thaliana, VERNALIZATION INSENSITIVE3 (VIN3) and its related plant homeodomain finger proteins act together with Polycomb Repressive Complex 2 to increase repressive histone marks at floral repressor loci, including FLOWERING LOCUS C (FLC) and its related genes, by vernalization. Here, we show that VIN3 family of proteins nonredundantly functions to repress different subsets of the FLC gene family during the course of vernalization. Each VIN3 family protein binds to modified histone peptides in vitro and directly associates with specific sets of FLC gene family chromatins in vivo to mediate epigenetic silencing. In addition, members of the FLC gene family are also differentially regulated during the course of vernalization to mediate proper vernalization response. Our results show that these two gene families cooperated during the course of evolution to ensure proper vernalization response through epigenetic changes.     

The most common chromosome aberration detected by high-resolution comparative genomic hybridization in vulvar intraepithelial neoplasia is not seen in vulvar squamous cell carcinoma

We analyzed genetic changes in condylomas (four cases), vulvar intraepithelial neoplasia I-III (VIN I-III, eleven cases), and primary vulvar squamous cell carcinomas (VSCC, ten cases) by high-resolution comparative genomic hybridization (HR-CGH) and flowcytometry. All samples were also human papilloma virus (HPV)-genotyped. Gain of chromosome 1, the aberration most often seen in VIN III (67%), was not seen in HPV-positive or -negative VSCCs (0%). Both VIN III and VSCC frequently showed gain of 3q (56 and 70%, respectively). The VIN III samples often demonstrated gain of 20q (56%) and 20p (44%), and the VSCC samples gain of 8q (60%), loss of 3p (50%), and 8p (40%). None of the four most frequent changes in the VSCC samples occurred exclusively in the HPV-positive or -negative samples. As expected, we did not find any cytogenetic changes in condylomas and nearly any changes in VIN I-II.     

Cloning and expression profiling polycomb gene VERNALIZATION INSENSITIVE 3 in tomato

VERNALIZATION INSENSITIVE 3 (VIN3) is a chromatin remodelling protein that is induced by low temperatures and is required for the vernalization response in Arabidopsis thaliana. VIN3 is one of the polycomb group (PcG) proteins, which mediates epigenetic repression of FLOWERING LOCUS C (FLC) in A. thaliana. Here, we present cloning, characterization, and expression of a putative SlVIN3 gene in tomato (Solanum lycopersicum L.) by isolating cDNA clones corresponding to SlVIN3 gene using primers designed based on conserved sequences between PcG genes in A. thaliana and tomato. The SlVIN3 cDNAs were cloned into a pBS plasmid and sequenced. Both 5′ and 3′ RACE were generated and sequenced. The flcDNA of 2 823 bp length for the SlVIN3 gene was composed of 5’UTR (336 bp), ORF (2 217 bp), and 3’UTR (270 bp). The translated ORF encoded a polypeptide of 739 amino acids. Alignment of deduced amino acids indicates that there are highly conserved regions between tomato SlVIN3 predicted protein and plant VIN3 gene family members. Both unrooted phylogenetic trees constructed using the maximum parsimony and maximum likelihood methods indicate that there is a close relationship between SlVIN3 predicted protein and VIN3 protein of Vitis vinifera. The expression of SlVIN3 gene remained high during floral organ differentiation and growth and decreased when the fruit started to develop.     

The PHD finger protein VRN5 functions in the epigenetic silencing of Arabidopsis FLC

Vernalization, the acceleration of flowering by the prolonged cold of winter, ensures that plants flower in favorable spring conditions. During vernalization in Arabidopsis, cold temperatures repress FLOWERING LOCUS C (FLC) expression in a mechanism involving VERNALIZATION INSENSITIVE 3 (VIN3), and this repression is epigenetically maintained by a Polycomb-like chromatin regulation involving VERNALIZATION 2 (VRN2), a Su(z)12 homolog, VERNALIZATION 1 (VRN1), and LIKE-HETEROCHROMATIN PROTEIN 1. In order to further elaborate how cold repression triggers epigenetic silencing, we have targeted mutations that result in FLC misexpression both at the end of the prolonged cold and after subsequent development. This identified VERNALIZATION 5 (VRN5), a PHD finger protein and homolog of VIN3. Our results suggest that during the prolonged cold, VRN5 and VIN3 form a heterodimer necessary for establishing the vernalization-induced chromatin modifications, histone deacetylation, and H3 lysine 27 trimethylation required for the epigenetic silencing of FLC. Double mutant and FLC misexpression analyses reveal additional VRN5 functions, both FLC-dependent and -independent, and indicate a spatial complexity to FLC epigenetic silencing with VRN5 acting as a common component in multiple pathways.     

Cytotoxic potentiation of vinblastine and paclitaxel by L-canavanine in human cervical cancer and hepatocellular carcinoma cells

BackgroundThe non-protein amino acid L-canavanine (L-CAV), found in several plants of the family Fabaceae is an antimetabolite which shows anticancer activity due to its ability to be incorporated into protein in the place of its analogue, L-arginine (L-ARG), leading to the alteration of the 3D conformation of newly synthesised proteins and usually a loss of their function.PurposeIn this study, the ability of L-CAV to potentiate the cytotoxicity of microtubule- targeting drugs used in the chemotherapy of cancer, vinblastine (VIN) and paclitaxel (PTX) was evaluated.Material and methodsThe following cancer cells grown in arginine-rich and arginine-free media were employed: HeLa, Hep G2 and SK-HEP-1. Drug combination experiment used a method based on the median-effect principle and mass-action law.ResultsWe observed that L-CAV, which is hardly toxic alone, potentiated the cytotoxicity of VIN and PTX in HeLa and hepatocellular carcinoma cells.ConclusionThis is the first study showing the cytotoxic potentiation of microtubule-targeting drugs by L-CAV. The mechanism of synergy and animal studies need to be investigated further to see whether L-CAV might become an adjuvant in cancer treatment.     

Vernalization-induced trimethylation of histone H3 lysine 27 at FLC is not maintained in mitotically quiescent cells

Vernalization promotes flowering in Arabidopsis through epigenetic repression of the floral repressor, FLOWERING LOCUS C (FLC). Vernalization, like other polycomb-mediated repression events, occurs in two stages; FLC repression is established at low temperatures, then maintained during subsequent growth at 22 degrees C. Low temperatures induce VIN3 activity, which is required for changes in histone modifications and the associated FLC repression. Plant polycomb proteins FIE, VRN2, CLF, and SWN, together with VIN3, form a complex that adds histone H3 lysine 27 methylation at FLC in vernalized plants. VRN1 and LHP1 are required for maintenance of FLC repression. Tissue must be undergoing cell division during low-temperature treatments for acceleration of flowering to occur. We show that low-temperature treatments repress FLC in cells that are not mitotically active, but this repression is not fully maintained. Trimethyl-lysine 27 (K27me3), is enriched at the start of the FLC gene during the cold, before spreading across the locus after vernalization. In the absence of DNA replication, K27me3 is added to chromatin at the start of FLC but is removed on return to 22 degrees C. This suggests that DNA replication is essential for maintenance of vernalization-induced repression of FLC.