Mitotic cyclins stimulate the activity of c-Myb-like factors for transactivation of G2/M phase-specific genes in tobacco
Mitotic cyclins stimulate the activity of c-Myb-like factors for transactivation of G2/M phase-specific genes in tobacco
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DOI：
10.1074/jbc.m403171200
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发表时间：
2004-07-30
影响因子：
4.8
通讯作者：
Machida, Y
中科院分区：
文献类型：
--
作者：
Araki, S;Ito, M;Machida, Y
Myb 转录因子在 Myb 结构域中包含三个不完美的重复序列，是 Myb 超家族的进化保守成员。具有三个重复序列的脊椎动物 Myb 蛋白（c-Myb、A-Myb 和 B-Myb）在细胞周期的 G(1)/S 转换中发挥重要作用。在植物中，这种类型的 Myb 蛋白通过激活或抑制细胞周期蛋白 B 基因和多种其他 G(2)/M 期特异性基因的转录来控制 G(2)/M 期。在烟草中，Myb 激活剂的两个基因 NtmybA1 和 NtmybA2 受到转录控制，并在 G(2)/M 期特异性表达。正如我们在此所示，除了转录水平的控制外，NtmybA2 的活性也在翻译后水平受到控制。我们发现 NtmybA2 的反式激活潜力受到位于其羧基末端的调节域的抑制，并且特定类别的细胞周期蛋白 A 和 B 可能通过缓解这种抑制作用来增强 NtmybA2 活性。 NtmybA2 中细胞周期蛋白依赖性激酶 (CDK) 的 20 个潜在磷酸化位点的突变阻止了细胞周期蛋白对 NtmybA2 活性的增强作用。重组 NtmybA2 在体外被从烟草 BY2 细胞制备的 CDK 级分磷酸化。 CDK 部分中 NtmybA2 的激酶活性在 BY2 细胞中受细胞周期调节，在 G(2)/M 期达到峰值，此时细胞周期蛋白 B 的转录物水平最高。综上所述，我们的数据表明 NtmybA2 在 G2/M 处被特定的细胞周期蛋白/CDK 复合物磷酸化，并且这种磷酸化消除了其 C 末端区域的抑制作用，从而在 G(2)/M 处特异性激活 NtmybA2。
Myb transcription factors, which contain three imperfect repeats in the Myb domain, are evolutionarily conserved members of the Myb superfamily. Vertebrate Myb proteins with three repeats, c-Myb, A-Myb, and B-Myb, play important roles at the G(1)/S transition in the cell cycle. In plants, this type of Myb protein controls the G(2)/M phase by activating or repressing the transcription of cyclin B genes and a variety of other G(2)/M phase-specific genes. In tobacco, two genes for Myb activators, NtmybA1 and NtmybA2, are transcriptionally controlled and are expressed specifically at the G(2)/M phase. As we showed here, in addition to the control at the transcriptional level, activity of NtmybA2 is also controlled at the post-translational level. We found that the transactivation potential of NtmybA2 is repressed by a regulatory domain located at its carboxyl terminus and that specific classes of cyclins A and B enhanced NtmybA2 activity possibly by relieving this inhibitory effect. Mutations at the 20 potential sites of phosphorylation by cyclin-dependent kinase (CDK) in NtmybA2 blocked the enhancing effects of the cyclins on NtmybA2 activity. Recombinant NtmybA2 was phosphorylated in vitro by a CDK fraction prepared from tobacco BY2 cells. The kinase activity for NtmybA2 in the CDK fraction was cell cycle-regulated in BY2 cells, peaking at the G(2)/M phase when the level of transcripts of cyclin B is maximal. Taken together, our data suggest that NtmybA2 is phosphorylated by a specific cyclin/CDK complex(es) at G2/M and that this phosphorylation removes the inhibitory effect of its C-terminal region, thereby activating NtmybA2 specifically at G(2)/M.