Originally published as Genetics Published Articles Ahead of Print on February 2, 2009.

Genetics, Vol. 181, 1321-1334, April 2009, Copyright © 2009
doi:10.1534/genetics.108.094839

Fission Yeast Rgf2p Is a Rho1p Guanine Nucleotide Exchange Factor Required for Spore Wall Maturation and for the Maintenance of Cell Integrity in the Absence of Rgf1p

Patricia García, Ignacio García1, Félix Marcos, Gorka Ruiz de Garibay2 and Yolanda Sánchez3

Instituto de Microbiología Bioquímica, CSIC/Universidad de Salamanca and Departamento de Microbiología y Genética, Universidad de Salamanca, Campus Miguel de Unamuno, 37007 Salamanca, Spain

3 Corresponding author: Instituto de Microbiología Bioquímica CSIC/Universidad de Salamanca, Edificio Departamental, Room 231, Campus Miguel de Unamuno, 37007 Salamanca, Spain.
E-mail: ysm{at}usal.es

Schizosaccharomyces pombe Rho1p is essential, directly activates β-1,3-glucan synthase, and participates in the regulation of morphogenesis. In S. pombe, Rho1p is activated by at least three guanine nucleotide exchange factors (GEFs): Rgf1p, Rgf2p, and Rgf3p. In this study we show that Rgf2p is a Rho1p GEF required for sporulation. The rgf2+ deletion did not affect forespore membrane formation and the nuclei were encapsulated properly. However, the mutant ascospores appeared dark and immature. The rgf2{Delta} zygotes were not able to release the ascospores spontaneously, and the germination efficiency was greatly reduced compared to wild-type (wt) spores. This phenotype resembles that of the mutants in bgs2+, which encodes a sporulation-specific glucan synthase subunit. In fact, glucan synthase activity was diminished in sporulating rgf2{Delta} diploids. Rgf2p also plays a role in β-glucan biosynthesis during vegetative growth. Overexpression of rgf2+ specifically increased GTP-bound Rho1p, caused changes in cell morphology, and elicited an increase in β-1,3-glucan synthase activity. Moreover, the simultaneous disruption of rgf1+ and rgf2+ was lethal and both Rgf1p and Rgf2p were able to partially substitute for each other. Our results suggest that Rgf1p and Rgf2p are alternative GEFs with an essential overlapping function in Rho1p activation during vegetative growth.