Stanhill A, Schick N, Engelberg D
Department of Biological Chemistry, The Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Mol Cell Biol. 1999 Nov;19(11):7529-38. doi: 10.1128/MCB.19.11.7529.
Haploid yeast cells are capable of invading agar when grown on rich media. Cells of the Sigma1278b genetic background manifest this property, whereas other laboratory strains are incapable of invasive growth. We show that disruption of the RAS2 gene in the Sigma1278b background significantly reduces invasive growth but that expression of a constitutively active Ras2p (Ras2(Val19)p) in this strain has a minimal effect on its invasiveness. On the other hand, expression of Ras2(Val19)p in another laboratory strain, SP1, rendered it invasive. These results suggest that a hyperactive Ras2 pathway induces invasive growth and that this pathway might be overactive in the Sigma1278b genetic background. Indeed, cells of the Sigma1278b are defective in the induction of stress-responsive genes, while their Gcn4 target genes are constitutively transcribed. This pattern of gene expression was previously shown to be associated with an active Ras/cyclic AMP (cAMP) pathway. We show that suppression of stress-related genes in Sigma1278b cells is a result of their inability to activate transcription through the stress response element (STRE). Disruption of RAS2, which abolished invasiveness, induced an increase in STRE activity. Further, in the SP1 genetic background, disruption of either the MSN2/4 genes (encoding activators of STRE) or the yAP-1 gene was sufficient to restore invasive growth in ras2Delta cells. We conclude that Ras2-mediated suppression of the stress response is sufficient to induce invasiveness. Accordingly, the fact that the stress response is suppressed in Sigma1278b background explains its invasiveness. It seems that invasiveness is a phenotype related to unregulated growth and is therefore manifested by cells harboring an overactive Ras/cAMP cascade. In this respect, invasiveness in yeast is reminiscent of the property of ras-transformed fibroblasts to invade soft agar.
单倍体酵母细胞在丰富培养基上生长时能够侵入琼脂。具有Sigma1278b遗传背景的细胞表现出这种特性，而其他实验室菌株则无法进行侵袭性生长。我们发现，在Sigma1278b背景下破坏RAS2基因会显著降低侵袭性生长，但在该菌株中组成型激活的Ras2p（Ras2(Val19)p）的表达对其侵袭性影响极小。另一方面，在另一个实验室菌株SP1中表达Ras2(Val19)p使其具有侵袭性。这些结果表明，过度活跃的Ras2途径诱导侵袭性生长，并且该途径在Sigma1278b遗传背景中可能过度活跃。实际上，Sigma1278b的细胞在应激反应基因的诱导方面存在缺陷，而它们的Gcn4靶基因则持续转录。这种基因表达模式先前已被证明与活跃的Ras/环磷酸腺苷（cAMP）途径相关。我们表明，Sigma1278b细胞中应激相关基因的抑制是由于它们无法通过应激反应元件（STRE）激活转录。消除侵袭性的RAS2破坏导致STRE活性增加。此外，在SP1遗传背景中，破坏MSN2/4基因（编码STRE的激活剂）或yAP-1基因足以恢复ras2Delta细胞的侵袭性生长。我们得出结论，Ras2介导的应激反应抑制足以诱导侵袭性。因此，在Sigma1278b背景中应激反应被抑制这一事实解释了其侵袭性。侵袭性似乎是一种与生长失控相关的表型，因此由具有过度活跃的Ras/cAMP级联反应的细胞表现出来。在这方面，酵母中的侵袭性让人联想到ras转化的成纤维细胞侵入软琼脂的特性。
