Yeast Lrg1p acts as a specialized RhoGAP regulating 1,3-β-glucan synthesis

Selection of an extragenic suppressor of fks1-1154 Delta fks2, mutations in the catalytic subunits of yeast 1,3-beta -glucan synthase (GS) conferring temperature-sensitivity, led to the LRG1 gene, which was originally identified as a LIM-RhoGAP homologous gene. Mutations in the LRG1 gene restore impaired 1,3-beta -glucan synthesis in the fks-1154 Delta fks2 mutant as well as that in rho1-2, a temperature-sensitive mutant of Rho-type GTPase that functions as a regulatory subunit of GS, Two-hybrid analyses of Lrg1p, which contains a sequence conserved among Rho GTPase-activating proteins (GAPs), revealed its specific interactions with the active form of Rho1p, Among eight potential yeast RhoGAPs, Lrg1p is the only member that negatively regulates GS activity: mutations in the rest of GAPs, including bem2, Delta bem3, Delta sac7, Delta bag7, Delta rga1, Delta rga2 and Delta rgd1, do not suppress impairment of 1,3-beta -glucan synthesis. Analyses of Mpk1p phosphorylation revealed the inability of Lrg1p to regulate the Pkc1p-MAP kinase cascade, a distinct Rho1p-regulating signalling pathway known to be affected by the GAPs, Bem2p and Sac7p, Thus, different groups of Rho1p GAPs control the activity of different Rho1p-effector proteins. Copyright (C) 2001 John Wiley & Sons, Ltd.