Trichoderma reesei rho3, a homologue of yeast RHO3, suppresses the growth defect of yeast sec15-1 mutation

The Trichoderma reesei gene, rho3, encoding the functional homologue of the Saccharomyces cerevisiae small GTP-binding protein Rho3p was cloned as a suppressor of the secretion-deficient mutation sec15-1 in yeast. The encoded protein showed 61% amino acid identity to the Rho3 protein. Rescue of the growth defect of a RHO3 disruption strain by an expression vector carrying rho3 cDNA confirmed the functional homology with the S. cerevisiae RHO3 gene. In addition, over-production of T. reesei RHOIII in this yeast strain appeared to improve the actin organization and chitin localization of the cells. Three putative mutant alleles of the T. reesei rho3 gene (rho3^Gly20Val, rho3^Thr25Asn, rho3^Asp126Ala) were introduced into the wild-type yeast, in yeast with sec15 mutation and in yeast with Rho3p depletion. Cells expressing rho3^Gly20Val displayed wild-type growth and those expressing rho3^Thr25Asn and rho3^Asp126Ala had a loss-of-function phenotype.