title: A Fungus-Specific Protein Domain Is Essential for RasA-Mediated Morphogenetic Signaling inAspergillus fumigatus.
abstract: Ras proteins function as conserved regulators of eukaryotic growth and differentiation and are essential signaling proteins orchestrating virulence in pathogenic fungi. Here, we report the identification of a novel N-terminal domain of the RasA protein in the filamentous fungusAspergillus fumigatus. Whereas this domain is absent in Ras homologs of higher eukaryotes, the N-terminal extension is conserved among fungi and is characterized by a short string of two to eight amino acids terminating in an invariant arginine. For this reason, we have termed the RasA N-terminal domain the invariant arginine domain (IRD). Through mutational analyses, the IRD was found to be essential for polarized morphogenesis and asexual development, with the invariant arginine residue being most essential. Although IRD truncation resulted in a nonfunctional Ras phenotype, IRD mutation was not associated with mislocalization of the RasA protein or significant changes in steady-state RasA activity levels. Mutation of the RasA IRD diminished protein kinase A (PKA) activation and resulted in decreased interaction with the Rho-type GTPase, Cdc42. Taken together, our findings reveal novel, fungus-specific mechanisms for Ras protein function and signal transduction.IMPORTANCEAspergillus fumigatusis an important fungal pathogen against which limited treatments exist. During invasive disease,A. fumigatushyphae grow in a highly polarized fashion, forming filaments that invade blood vessels and disseminate to distant sites. Once invasion and dissemination occur, mortality rates are high. We have previously shown that the Ras signaling pathway is an important regulator of the hyphal growth machinery supporting virulence inA. fumigatus. Here, we show that functional Ras signaling inA. fumigatusrequires a novel, fungus-specific domain within the Ras protein. This domain is highly conserved among fungi, yet absent in higher eukaryotes, suggesting a potentially crucial difference in the regulation of Ras pathway activity between the human host and the fungal pathogen. Exploration of the mechanisms through which this domain regulates signaling could lead to novel antifungal therapies specifically targeting fungal Ras pathways.
authors.affiliation: Department of Clinical Pharmacy, University of Tennessee Health Science Center, Memphis, Tennessee, USA.
authors.forename: Qusai
authors.initials: Q
authors.lastname: Al Abdallah
authors.affiliation: Department of Microbiology and Immunology, University of South Alabama, Mobile, Alabama, USA.
authors.forename: Tiffany S
authors.initials: TS
authors.lastname: Norton
authors.affiliation: Department of Microbiology and Immunology, University of South Alabama, Mobile, Alabama, USA.
authors.forename: Amy M
authors.initials: AM
authors.lastname: Hill
authors.affiliation: Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile, Alabama, USA.
authors.forename: Lawrence L
authors.initials: LL
authors.lastname: LeClaire
authors.affiliation: Department of Clinical Pharmacy, University of Tennessee Health Science Center, Memphis, Tennessee, USA.
authors.forename: Jarrod R
authors.initials: JR
authors.lastname: Fortwendel
keywords: Aspergillus; Ras; actin; morphogenesis; signal transduction