The yeast Ras/cyclic AMP pathway induces invasive growth by suppressing the cellular stress response

Haploid yeast cells are capable of invading agar when grown on rich media. Cells of the Sigma 1278b genetic background manifest this property, whereas other laboratory strains are incapable of invasive growth. We show that di sruption of the RAS2 gene in the Sigma 1278b background significantly reduc es invasive growth but that expression of a constitutively active Ras2p (Ra s2(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 S igma 1278b genetic background. Indeed, cells of the Sigma 1278b are defecti ve in the induction of stress-responsive genes, while their Gcn4 target gen es are constitutively transcribed. This pattern of gene expression was prev iously shown to be associated with an active Ras/cyclic AMP (cAMP) pathway. We show that suppression of stress-related genes in Sigma 1278b cells is a result of their inability to activate transcription through the stress res ponse element (STRE). Disruption of RAS2, which abolished invasiveness, ind uced 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 ras2 Delta cells. W e conclude that Ras2-mediated suppression of the stress response is suffici ent to induce invasiveness. Accordingly, the fact that the stress response is suppressed in Sigma 1278b background explains its invasiveness. It seems that invasiveness is a phenotype related to unregulated growth and is ther efore manifested by cells harboring an overactive Ras/cAMP cascade. In this respect, invasiveness in yeast is reminiscent of the property of ras-trans formed fibroblasts to invade soft agar.