Schouten J, de Kam R J, Fetter K, Hoge J H
Institute of Molecular Plant Sciences, Leiden University, Clusius Laboratory, The Netherlands.
Mol Gen Genet. 2000 Mar;263(2):309-19. doi: 10.1007/pl00008682.
The timed destruction of cell cycle regulatory proteins is of key importance in controlling cell cycle progression in eukaryotes. Recently, Skp1 from yeast (Saccharomyces cerevisiae) was shown to play an important role in the ubiquitin-mediated proteolysis of these proteins via the Skp1-Cdc53-F-box (SCF) pathway. Here we describe the fortuitous cloning of cDNAs for two Skp1 homologues from the plant Arabidopsis thaliana on account of their ability to activate reporter gene expression in yeast directed by the cyt-1 element from the promoter of the Agrobacterium tumefaciens T-cyt gene, which is essential for expression of the gene in plants. This element is strikingly similar in sequence to the binding site for the yeast Migl protein, a transcriptional repressor of genes involved in the utilisation of carbohydrates other than glucose. We report that Mig1 protein binds to the cyt-1 element with similar specificity as a previously described plant nuclear protein factor, and that the cyt-1 element is a target for an unknown yeast transcriptional activator when Mig1 itself is inactivated. Interestingly, our data further indicate that A. thaliana Skp1 inactivates Mig1 by destabilising the yeast F-box protein Grr1, which is required for cyclin degradation and is thus involved in control of the cell cycle, and for glucose-regulated gene repression. Our results suggest that the plant counterpart of yeast Skp1 is probably also instrumental in ubiquitin-mediated proteolysis of specific proteins via an SCF-like pathway.
细胞周期调节蛋白的定时降解对于控制真核生物的细胞周期进程至关重要。最近，来自酵母（酿酒酵母）的Skp1被证明在通过Skp1-Cdc53-F-box（SCF）途径对这些蛋白进行泛素介导的蛋白水解过程中发挥重要作用。在此，我们描述了从植物拟南芥中偶然克隆到两个Skp1同源物的cDNA，这是由于它们能够激活由根癌土壤杆菌T-cyt基因启动子的cyt-1元件指导的酵母中的报告基因表达，该元件对于该基因在植物中的表达至关重要。该元件在序列上与酵母Migl蛋白的结合位点惊人地相似，Migl蛋白是参与利用除葡萄糖以外的碳水化合物的基因的转录阻遏物。我们报道Migl蛋白以与先前描述的植物核蛋白因子相似的特异性结合到cyt-1元件上，并且当Migl本身失活时，cyt-1元件是未知酵母转录激活因子的靶标。有趣的是，我们的数据进一步表明拟南芥Skp1通过使酵母F-box蛋白Grr1不稳定来使Migl失活，Grr1是细胞周期蛋白降解所必需的，因此参与细胞周期的控制以及葡萄糖调节的基因抑制。我们的结果表明，酵母Skp1的植物对应物可能也通过类似SCF的途径在泛素介导的特定蛋白的蛋白水解中起作用。
