A Rac homolog functions downstream of Ras1 to control hyphal differentiation and high-temperature growth in the pathogenic fungus Cryptococcus neoformans.
The Cryptococcus neoformans Ras1 protein serves as a central regulator for several signaling pathways. Ras1 controls the induction of the mating pheromone response cascade as well as a distinct signaling pathway that allows this pathogenic fungus to grow at human physiological temperature. To characterize elements of the Ras1-dependent high-temperature growth pathway, we performed a multicopy suppressor screen, identifying genes whose overexpression allows the ras1 mutant to grow at 37 degrees C. Using this genetic technique, we identified a C. neoformans gene encoding a Rac homolog that suppresses multiple ras1 mutant phenotypes. Deletion of the RAC1 gene does not affect high-temperature growth. However, a rac1 mutant strain demonstrates a profound defect in haploid filamentation as well as attenuated mating. In a yeast two-hybrid assay, Rac1 physically interacts with the PAK kinase Ste20, which similarly regulates hyphal formation in this fungus. Similar to Rac1, overexpression of the STE20alpha gene also restores high-temperature growth to the ras1 mutant. These results support a model in which the small G protein Rac1 acts downstream of Ras proteins and coordinately with Ste20 to control high-temperature growth and cellular differentiation in this human fungal pathogen.
Vallim
Marcelo A
MA
Department of Medicine, Duke University Medical Center (DUMC) 3355, Durham, NC 27710, USA.
Nichols
Connie B
CB
Fernandes
Larissa
L
Cramer
Kari L
KL
Alspaugh
J Andrew
JA
D001483:Base Sequence; D003455:Cryptococcus neoformans / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000472:pathogenicity / Q000502:physiology*; D004271:DNA, Fungal; D005656:Fungal Proteins; D015966:Gene Expression Regulation, Fungal; D005800:Genes, Fungal; D016681:Genome, Fungal; D006358:Hot Temperature; D025301:Hyphae / Q000254:growth & development*; D009154:Mutation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D020798:Two-Hybrid System Techniques; D054462:p21-Activated Kinases; D020744:rac GTP-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D018631:ras Proteins / Q000235:genetics / Q000378:metabolism*
D004271:DNA, Fungal; D005656:Fungal Proteins; C515903:PAK1 protein, human; D017346:Protein Serine-Threonine Kinases; D054462:p21-Activated Kinases; D020744:rac GTP-Binding Proteins; D018631:ras Proteins