Date Published: September 20, 2016
Publisher: Public Library of Science
Author(s): Bastian Jöhnk, Özgür Bayram, Anja Abelmann, Thorsten Heinekamp, Derek J. Mattern, Axel A. Brakhage, Ilse D. Jacobsen, Oliver Valerius, Gerhard H. Braus, Xiaorong Lin.
F-box proteins share the F-box domain to connect substrates of E3 SCF ubiquitin RING ligases through the adaptor Skp1/A to Cul1/A scaffolds. F-box protein Fbx15 is part of the general stress response of the human pathogenic mold Aspergillus fumigatus. Oxidative stress induces a transient peak of fbx15 expression, resulting in 3x elevated Fbx15 protein levels. During non-stress conditions Fbx15 is phosphorylated and F-box mediated interaction with SkpA preferentially happens in smaller subpopulations in the cytoplasm. The F-box of Fbx15 is required for an appropriate oxidative stress response, which results in rapid dephosphorylation of Fbx15 and a shift of the cellular interaction with SkpA to the nucleus. Fbx15 binds SsnF/Ssn6 as part of the RcoA/Tup1-SsnF/Ssn6 co-repressor and is required for its correct nuclear localization. Dephosphorylated Fbx15 prevents SsnF/Ssn6 nuclear localization and results in the derepression of gliotoxin gene expression. fbx15 deletion mutants are unable to infect immunocompromised mice in a model for invasive aspergillosis. Fbx15 has a novel dual molecular function by controlling transcriptional repression and being part of SCF E3 ubiquitin ligases, which is essential for stress response, gliotoxin production and virulence in the opportunistic human pathogen A. fumigatus.
The ubiquitin 26S proteasome system (UPS) controls the life span of specific regulatory proteins, which are required for coordinated development, signal transduction and DNA maintenance. Target proteins are linked to ubiquitin by the sequential action of E1, E2 and E3 enzymes. A crucial step during this enzymatic cascade is carried out by E3 ubiquitin ligases, which recognize their specific substrate and catalyze the transfer of ubiquitin. SCF-complexes are multi-subunit E3 enzymes consisting of three major subunits (Cul1, Skp1 and Rbx1), which form the core enzyme and an exchangeable set of substrate-specific adaptors called F-box proteins [1,2]. The F-box domain of these adaptors is an N-terminal binding site of approximately 45 amino acids. It binds to the Skp1 linker to connect to Cul1. The human genome encodes 69 F-box proteins and defects in F-box mediated ubiquitination are associated with various diseases like diabetes, Parkinson or cancer [3–5]. Only little is known about the role of F-box proteins in virulence of fungal pathogens, though fungal F-box proteins play important roles for cellular development, transcription, signal transduction and nutrient sensing [6–8].
Aspergillus fumigatus is the most prevalent cause for pulmonary infections in immunocompromised patients. High thermo- and oxidative stress tolerance, toxic metabolites and a versatile metabolism allow A. fumigatus to colonize host tissue . We identified the fungal-specific F-box protein Fbx15, which is not required for vegetative growth in the absence of stress, as key determinant for stress response, controlled gliotoxin production and virulence. A novel dual molecular function was discovered for Fbx15. Fbx15 can be part of an SCF E3 ubiquitin ligase complex and in addition controls nuclear localization of SsnF as transcriptional repressor subunit. Fbx15 levels are transcriptionally regulated and Fbx15 location in either the nucleus or the cytoplasm is determined by phosphorylation and dephosphorylation, respectively. Fbx15 is a potential target for antifungal drugs, because it is essential for A. fumigatus virulence.