CELL-WALL INTEGRITY MODULATES RHO1 ACTIVITY VIA THE EXCHANGE FACTOR ROM2

The essential phosphatidylinositol kinase homologue TOR2 of Saccharomy ces cerevisiae controls the actin cytoskeleton by activating a GTPase switch consisting of RHO1 (GTPase), ROM2 (GEF) and SAC7 (GAP), We have identified two mutations, rot1-1 anti rot2-1, that suppress the loss of TOR2 and are synthetic-lethal, The wild-type ROT1 and ROT2 genes an d a multicopy suppressor, BIG1, were isolated by their ability to resc ue the rot1-1 rot2-1 double mutant, ROT2 encodes glucosidase II, and R OT1 and BIG1 encode novel proteins, We present evidence that cell wall defects activate RHO1, First, rot1, rot2, big1, cwh41, gas1 and fks1 mutations all confer cell wall defects and suppress tor2(ts). Second, destabilizing the cell wall by supplementing the growth medium with 0. 005% SDS also suppresses a tor2(ts) mutation, Third, disturbing the ce ll wall with SDS or a rot1, rot2, big1, cwh41, gas1 or fks1 mutation i ncreases GDP/GTP exchange activity toward RHO1, These results suggest that cell wall defects suppress a tor2 mutation by activating RHO1 ind ependently of TOR2, thereby inducing TOR2-independent polarization of the actin cytoskeleton and cell wall synthesis, Activation of RHO1, a subunit of the cell wall synthesis enzyme glucan synthase, by a sell w all alteration would ensure that cell wall synthesis occurs only when and where needed, The mechanism of RHO1 activation by a cell wall alte ration is via the exchange factor ROM2 and could be analogous to signa lling by integrin receptors in mammalian cells.