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MSI1 suppresses hyperactive RAS via the cAMP-dependent protein kinase and independently of chromatin assembly
1Department of Developmental and Molecular Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
Abstract:
RAS hyperactivation in the yeast Saccharomyces cerevisiae leads to multiple nutritional growth defects associated with overstimulation of the cAMP signaling pathway. Hyperactive RAS can be suppressed by overexpression of MSI1, a subunit of chromatin assembly factor-1 (yCAF-1). MSI1 overexpression suppresses phenotypes induced by increased cAMP content in multiple genetic backgrounds. However, MSI1 does not inhibit cAMP synthesis or total cellular cAMP-dependent protein kinase (PKA) activity, nor does MSI1 stimulate expression of several cAMP-repressible genes critical for the acquisition of thermotolerance in the stationary phase. Our analysis indicates that yCAF-1 is dispensable for inhibition of hyperactive RAS by MSI1. We demonstrate that in the presence of the PKA regulatory subunit, BCY1, MSI1 inhibits phenotypes of a mutationally activated PKA catalytic subunit. These observations indicate that MSI1 affects PKA function in a BCY1-dependent manner via mechanisms other than direct overall inhibition of PKA catalytic activity. MSI1 appears to provide two distinct roles in chromatin modeling as a component of yCAF-1, and in the inhibition of RAS signaling by modulating PKA.
Insights
Overexpression of MSI1 suppresses RAS hyperactivation in yeast by modulating protein kinase A (PKA) activity, independent of cAMP levels. This suggests MSI1 has dual roles in chromatin and RAS signaling pathways.
Area of Science:
- Molecular biology
- Cell signaling
- Yeast genetics
Background:
- RAS hyperactivation in Saccharomyces cerevisiae causes growth defects due to elevated cAMP signaling.
- MSI1, a subunit of chromatin assembly factor-1 (yCAF-1), has been identified as a suppressor of these RAS-induced phenotypes.
Purpose of the Study:
- To investigate the mechanism by which MSI1 suppresses RAS hyperactivation.
- To determine if MSI1's function is linked to cAMP synthesis, PKA activity, or chromatin assembly.
Main Methods:
- Genetic analysis in Saccharomyces cerevisiae.
- Phenotypic analysis of MSI1 overexpression under conditions of RAS hyperactivation.
- Assays measuring cAMP levels and protein kinase A (PKA) activity.
- Examination of BCY1-dependent PKA regulation.
Main Results:
- MSI1 overexpression suppressed RAS hyperactivation phenotypes without affecting cAMP synthesis or overall PKA activity.
- The suppressive effect of MSI1 was dependent on the presence of BCY1, the PKA regulatory subunit.
- MSI1 inhibited phenotypes of a constitutively active PKA catalytic subunit in a BCY1-dependent manner.
- Chromatin assembly factor-1 (yCAF-1) was found to be dispensable for MSI1's RAS-suppressive function.
Conclusions:
- MSI1 inhibits RAS hyperactivation through a mechanism that modulates PKA function in a BCY1-dependent manner, rather than by broadly inhibiting PKA activity or cAMP synthesis.
- MSI1 plays distinct roles in both chromatin modeling and RAS/PKA signaling pathway regulation.