AtSIG6, a plastid sigma factor from Arabidopsis, reveals functional impact of cpCK2 phosphorylation.
AtSIG6, a plastid sigma factor from Arabidopsis, reveals functional impact of cpCK2 phosphorylation.
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DOI：
10.1111/j.1365-313x.2010.04138.x
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发表时间：
2010-04
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通讯作者：
Link G
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作者：
Schweer J;Türkeri H;Link B;Link G
叶绿体含有西格玛因子，即启动子结合和转录启动的基因调控蛋白。尽管植物西格玛因子在物理和功能上与原核生物相似，但它们有不同的特征：最显著的是存在一个由其N-末端部分组成的可变额外序列。这种独特的结构反映在功能上的差异，包括由细胞器蛋白激酶(S)控制的磷酸化，与核浆酶密切相关，而不是细菌型酶。特别是，cpCK2，一种以胞体为靶标的核编码酪蛋白激酶2，被认为是植物Sigma因子磷酸化和转录调控(EUR。J.生物化学。269,2002,3329；Planta，219,2004,298)。虽然这一概念主要基于生化证据和体外系统，但最近获得的拟南芥sigma基因敲除系可与完整和突变的sigma cDNA互补，为体内转录调控机制的研究开辟了新的策略。以拟南芥Sigma因子6(AtSIG6)为范例，我们提出的数据表明：(I)该因子是cpCK2在体外和体内调节磷酸化的底物；(Ii)SIG6的cpCK2磷酸化发生在多个位点，其对视觉和/或分子表型的影响可能有很大不同；(Iii)在体内使用Ser174定义的可能最关键的cpCK2位点需要在n+3丝氨酸残基Ser177处(预)磷酸化，指向能够产生功能性cpCK2底物位点的‘探路者’激酶活性。
Plastids contain sigma factors, i.e. gene-regulatory proteins for promoter binding and transcription initiation. Despite the physical and functional similarity shared with their prokaryotic counterparts, the plant sigma factors have distinguishing features: most notably the existence of a variable extra sequence comprising their N-terminal portions. This distinct architecture is reflected by functional differences, including phosphorylation control by organellar protein kinase(s) closely related to nucleocytosolic, rather than bacterial-type, enzymes. In particular, cpCK2, a nuclear-coded plastid-targeted casein kinase 2, has been implicated as a key component in plant sigma factor phosphorylation and transcriptional regulation (Eur. J. Biochem. 269, 2002, 3329; Planta, 219, 2004, 298). Although this notion is based mainly on biochemical evidence and in vitro systems, the recent availability of Arabidopsis sigma knock-out lines for complementation by intact and mutant sigma cDNAs has opened up new strategies for the study of transcription regulatory mechanisms in vivo. Using Arabidopsis sigma factor 6 (AtSIG6) as a paradigm, we present data suggesting that: (i) this factor is a substrate for regulatory phosphorylation by cpCK2 both in vitro and in vivo; (ii) cpCK2 phosphorylation of SIG6 occurs at multiple sites, which can widely differ in their effect on the visual and/or molecular phenotype; (iii) in vivo usage of the perhaps most critical cpCK2 site defined by Ser174 requires (pre-)phosphorylation at the n + 3 serine residue Ser177, pointing to ‘pathfinder’ kinase activity capable of generating a functional cpCK2 substrate site.