Originally published as Genetics Published Articles Ahead of Print on February 22, 2010.

Genetics, Vol. 185, 165-176, May 2010, Copyright © 2010
doi:10.1534/genetics.110.114165

Regulation of Septum Formation by the Bud3–Rho4 GTPase Module in Aspergillus nidulans

Haoyu Si*, Daniela Justa-Schuch{dagger}, Stephan Seiler{dagger} and Steven D. Harris*,1

* Department of Plant Pathology and Center for Plant Science Innovation, University of Nebraska, Lincoln, Nebraska 68588-0660 and {dagger} Institut für Mikrobiologie und Genetik, Universität Göttingen, D-37077 Göttingen, Germany

1 Corresponding author: Center for Plant Science Innovation, E126 Beadle Center, University of Nebraska, Lincoln, NE 68588-0660.
E-mail: sharri1{at}unlnotes.unl.edu

The ability of fungi to generate polarized cells with a variety of shapes likely reflects precise temporal and spatial control over the formation of polarity axes. The bud site selection system of Saccharomyces cerevisiae represents the best-understood example of such a morphogenetic regulatory system. However, the extent to which this system is conserved in the highly polarized filamentous fungi remains unknown. Here, we describe the functional characterization and localization of the Aspergillus nidulans homolog of the axial bud site marker Bud3. Our results show that AnBud3 is not required for polarized hyphal growth per se, but is involved in septum formation. In particular, our genetic and biochemical evidence implicates AnBud3 as a guanine nucleotide exchange factor for the GTPase Rho4. Additional results suggest that the AnBud3–Rho4 module acts downstream of the septation initiation network to mediate recruitment of the formin SepA to the site of contractile actin ring assembly. Our observations provide new insight into the signaling pathways that regulate septum formation in filamentous fungi.