Functional studies on Ira proteins, the neurofibromin homologs in yeast

In S. cerevisiae, PKA pathway controls metabolism, stress resistance and proliferation. PKA is regulated both by Gpr1/Gpa2 system and the small Ras proteins whose activity is finely controlled by Cdc25/Sdc25 guanine nucleotide exchange factors and Ira GTPase activating proteins. Ira1 and Ira2 are homologs of human neurofibromin 1 (NF1) protein, whose mutations were found to be involved in human neurofibromatosis. The yeast RasGAP proteins share similar aminoacids sequence and have related, but not identical,functions. Scaffolding functions associated with the N-terminal non-catalytic large regions of Ira proteins have not been completely characterized, and the suggested feedback regulation improving RasGAP activity is still unclear at the molecular level. The effects of disjunction of Ira proteins functional regions were investigated, by analyzing strains deprived either of the RasGAP domain or of the entire RasGAP protein, or by expressing the isolated RasGAP domain either in the presence or in the absence of the N-terminal region. Surprisingly, uncorrelated effects on cell size and heat shock sensitivity were observed, although both of these traits are classically reported as PKA activity related phenotypes. Finally, the effects on Ras complex components localization inside the cell were evaluated.