MSI1 suppresses hyperactive RAS via the cAMP-dependent protein kinase and independently of chromatin assembly factor-1

RAS hyperactivation in the yeast Saccharomyces cerevisiae leads to multiple nutritional growth defects associated with overstimulation of the cAMP sig naling pathway. Hyperactive RAS can be suppressed by overexpression of MSI1 , a subunit of chromatin assembly factor-1 (yCAF-1). MSI1 overexpression su ppresses phenotypes induced by increased cAMP content in multiple genetic b ackgrounds. However, MSI1 does not inhibit cAMP synthesis or total cellular cAMP-dependent protein kinase (PKA) activity, nor does MSI1 stimulate expr ession of several cAMP-repressible genes critical for the acquisition of th ermotolerance in the stationary phase. Our analysis indicates that yCAF-1 i s dispensable for inhibition of hyperactive RAS by MSI1. We demonstrate tha t in the presence of the PKA regulatory subunit, BCY1, MSI1 inhibits phenot ypes of a mutationally activated PKA catalytic subunit. These observations indicate that MSI1 affects PKA function in a BCY1-dependent manner via mech anisms other than direct overall inhibition of PKA catalytic activity. MSI1 appears to provide two distinct roles - in chromatin modeling as a compone nt of yCAF-1, and in the inhibition of RAS signaling by modulating PKA.