title: Cla4 kinase triggers destruction of the Rac1-GEF Cdc24 during polarized growth in Ustilago maydis.
abstract: Dimorphic switching from budding to filamentous growth is a characteristic feature of many pathogenic fungi. In the fungal model organism Ustilago maydis polarized growth is induced by the multiallelic b mating type locus and requires the Rho family GTPase Rac1. Here we show that mating type-induced polarized growth involves negative feedback regulation of the Rac1-specific guanine nucleotide exchange factor (GEF) Cdc24. Although Cdc24 is essential for polarized growth, its concentration is drastically diminished during filament formation. Cdc24 is part of a protein complex that also contains the scaffold protein Bem1 and the PAK kinase Cla4. Activation of Rac1 results in Cla4-dependent degradation of the Rac1-GEF Cdc24, thus creating a regulatory negative feedback loop. We generated mutants of Cdc24 that are resistant to Cla4-dependent destruction. Expression of stable Cdc24 variants interfered with filament formation, indicating that negative feedback regulation of Cdc24 is critical for the establishment of polarized growth.
authors.affiliation: Department of Biology, University of Marburg, 35032 Marburg, Germany.
authors.forename: Sonja Helene
authors.initials: SH
authors.lastname: Frieser
authors.forename: Andrea
authors.initials: A
authors.lastname: Hlubek
authors.forename: Björn
authors.initials: B
authors.lastname: Sandrock
authors.forename: Michael
authors.initials: M
authors.lastname: Bölker
mesh_terms: D000595:Amino Acid Sequence; D015536:Down-Regulation; D004789:Enzyme Activation; D025461:Feedback, Physiological; D005656:Fungal Proteins / Q000378:metabolism*; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D025301:Hyphae / Q000201:enzymology / Q000254:growth & development; D008969:Molecular Sequence Data; D009024:Morphogenesis; D046912:Multiprotein Complexes / Q000378:metabolism; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D059748:Proteolysis; D014588:Ustilago / Q000201:enzymology* / Q000254:growth & development; D020830:rac1 GTP-Binding Protein / Q000378:metabolism*
chemical_list: D005656:Fungal Proteins; D020662:Guanine Nucleotide Exchange Factors; D046912:Multiprotein Complexes; D017346:Protein Serine-Threonine Kinases; D020830:rac1 GTP-Binding Protein