Regulation of Aspergillus nidulans CreA-mediated catabolite repression by the F-box proteins Fbx23 and Fbx47

The attachment of one or more ubiquitin molecules by SCF (Skp-Cullin-Fbox) complexes to protein substrates targets them for subsequent degradation by the 26S proteasome, allowing the control of numerous cellular processes. Glucose-mediated signalling and subsequent carbon catabolite repression (CCR) are processes relying on the functional regulation of target proteins, ultimately controlling the utilization of this carbon source. In the filamentous fungus Aspergillus nidulans, CCR is mediated by the transcription factor CreA which modulates the expression of genes encoding biotechnologically-relevant enzymes. Although CreA-mediated repression of target genes has been extensively studied, less is known about the regulatory pathways governing CCR and this work aimed at further unravelling these events. The Fbox protein Fbx23 was identified as being involved in CCR and the Δfbx23 mutant presented impaired xylanase production in repressing (glucose) and de-repressing (xylan) conditions. Mass spectrometry showed that Fbx23 is part of a SCF ubiquitin ligase complex that is bridged via the protein kinase GskA to the CreA-SsnF-RcoA repressor complex, resulting in the degradation of the latter in de-repressing conditions. Upon the addition of glucose, CreA dissociates from the ubiquitin ligase complex and is transported into the nucleus. Furthermore, casein kinase subunits A and B are important for CreA function during glucose signalling although the exact role of phosphorylation in CCR remains to be determined. The addiction of glucose makes the protein kinase GskA to leave the nucleus and lose the interaction with Crea-SsnF-RcoA repressor complex, the deletion of GskA shows a sick phenotype and impaired hydrolytic enzymes secretion. In summary, this study unravelled novel mechanistic details underlying CreA-mediated CCR and provides a solid basis for studying additional factors involved in carbon source utilization which could prove useful for biotechnological applications.

Keywords: CCR, F-box, SCF complex, CreA, GskA, CkiA, CkiB, xylanase.

Financial support: FAPESP and CNPq, Brazil