Fission yeast Rho GAPs Rga7 and Rga8 regulate cell integrity and cytokinesis

Abstract

Rga7 and Rga8 are two of the nine Rho GTPase-activating proteins (GAPs) present in fission yeast. These GAPs are responsible for the inactivation of Rho GTPases, which are involved in the regulation of the cytoskeleton and many biological processes in eukaryotic cells. Rga7 is a GAP for Rho2-GTPase which acts upstream of Pmk1, a MAPK involved in the regulation ofthe cell integrity and cell separation. Rga8 is a GAP for Rho1 which is the major activator of the cell wall synthesis. Both GAPs contain and F-BAR doma in at the protein N-terminus and localize to the cell division site and to the cell tips. Mutant cells lacking Rga7 display cytokinesis defects that are aggravated by the simultaneous absence of Rga8 and we have found that Rga7 and Rga8 coimmunoprecipitate. Cells lacking Rga7 also have cell integrity defects and are hypersensitive to the calcineurin inhibitor FK506. The Rga7 GAP domain is necessary for the cell growth in the presence of this drug. By contrast, rga8Δ cells are not sensitive to FK506, and in these cells the Rga7 GAP doma in is not required to allow cell growth in medium supplemented with FK506. lnterestingly, MAPK cell integrity pathway, which might have a function antagonist to calcineurin, is activated in both rga8Δ and rga7Δ cells. Elimination of the MAPK Pmk1 completely suppresses cell integrity defects of rga7Δ mutant cells but increases the proportion of cells with septation defects. Two hybrid screening detected physical interaction between Rga7 and Rho2, as expected, and between Rga7 and Pek1, the MAPKK which acts as an activator of the Pmk1 MAPK signaling. All together, these data suggest a possible role of fission yeast Rga7 and Rga8 in cell integrity and cytokinesis that can be due to the regulation of the MAPK pathway, in both Rho2 dependent and independent ways.