Regulation of cell separation in the dimorphic fungus Ustilago maydis

Abstract

Summary During its haploid phase the dimorphic fungus Ustilago maydis grows vegetatively by budding. We have identified two genes, don1 and don3, which control the separation of mother and daughter cells. Mutant cells form tree‐like clusters in liquid culture and grow as ring‐like (donut‐shaped) colonies on solid medium. In wild‐type U. maydis cells, two distinct septa are formed during cytokinesis and delimit a fragmentation zone. Cells defective for either don1 or don3 display only a single septum and fail to complete cell separation. don1 encodes a guanine nucleotide exchange factor (GEF) of the Dbl family specific for Rho/Rac GTPases. Don3 belongs to the germinal‐centre‐kinase (GC) subfamily of Ste20‐like protein kinases. We have isolated the U. maydis homologues of the small GTP binding proteins Rho2, Rho3, Rac1 and Cdc42. Out of these, only Cdc42 interacts specifically with Don1 and Don3 in the yeast two‐hybrid system. We propose that Don1 and Don3 regulate the initiation of the secondary septum, which is required for proper cell separation.

Polarity proteins Bem1 and Cdc24 are components of the filamentous fungal NADPH oxidase complex

“…Cdc42, the yeast homolog of RacA, is a substrate of Cdc24 (24). In U. maydis, Don1 was identified as the GEF for Cdc42, and Cdc24 is proposed as the GEF for RacA to control the processes of cell septation and polarized growth, respectively (18,25,26). We previously showed that interaction of the GTP bound form of RacA with NoxR is essential for activation of the fungal Nox (13,16).…”

Cdc42 and the Ste20-like kinase Don3 act independently in triggering cytokinesis in Ustilago maydis

“…Although Don3 belongs to the GCK subfamily of Ste20-like kinases that lack a CRIB domain, epistasis analysis and yeast two-hybrid-studies suggested that its activity depends on its interaction with Cdc42 (Mahlert et al, 2006;Weinzierl et al, 2002). Accordingly, U. maydis don3 mutants display a cellseparation defect indistinguishable from that of cdc42 mutant strains (Mahlert et al, 2006).…”

“…The U. maydis strain Bub8 (Schulz et al, 1990) was used as wild-type background for all strains created in this manuscript. Bub8⌬don3 has been described previously (Weinzierl et al, 2002).…”

“…The mitotic exit network (MEN) and the septation initiation network (SIN), respectively, coordinate the onset of septation and cytokinesis with the exit from the mitotic cell cycle (Krapp et al, 2004;Simanis, 2003;Walther and Wendland, 2003). In the dimorphic basidiomycete Ustilago maydis, the GTP-binding protein Cdc42 together with its activator Don1 and the Ste20-like kinase Don3 are essential for the formation of a secondary septum, which is required for proper cytokinesis (Mahlert et al, 2006;Weinzierl et al, 2002). Deletion of either of don1, cdc42 or don3 interferes with cell separation resulting in the formation of large clusters of cells that are connected by their primary septum Weinzierl et al, 2002).…”

“…In the dimorphic basidiomycete Ustilago maydis, the GTP-binding protein Cdc42 together with its activator Don1 and the Ste20-like kinase Don3 are essential for the formation of a secondary septum, which is required for proper cytokinesis (Mahlert et al, 2006;Weinzierl et al, 2002). Deletion of either of don1, cdc42 or don3 interferes with cell separation resulting in the formation of large clusters of cells that are connected by their primary septum Weinzierl et al, 2002). Don3 belongs to the germinal centre kinase (GCK) subfamily of Ste20-like kinases.…”