Integration of Fungus-Specific CandA-C1 into a Trimeric CandA Complex Allowed Splitting of the Gene for the Conserved Receptor Exchange Factor of CullinA E3 Ubiquitin Ligases in Aspergilli

Abstract

E3 cullin-RING ubiquitin ligase (CRL) complexes recognize specific substrates and are activated by covalent modification with ubiquitin-like Nedd8. Deneddylation inactivates CRLs and allows Cand1/A to bind and exchange substrate recognition subunits. Human as well as most fungi possess a single gene for the receptor exchange factor Cand1, which is split and rearranged in aspergilli into two genes for separate proteins. Aspergillus nidulans CandA-N blocks the neddylation site, and CandA-C inhibits the interaction to the adaptor/substrate receptor subunits similar to the respective N-terminal and C-terminal parts of single Cand1. The pathogen Aspergillus fumigatus and related species express a CandA-C with a 190-amino-acid N-terminal extension domain encoded by an additional exon. This extension corresponds in most aspergilli, including A. nidulans, to a gene directly upstream of candA-C encoding a 20-kDa protein without human counterpart. This protein was named CandA-C1, because it is also required for the cellular deneddylation/neddylation cycle and can form a trimeric nuclear complex with CandA-C and CandA-N. CandA-C and CandA-N are required for asexual and sexual development and control a distinct secondary metabolism. CandA-C1 and the corresponding domain of A. fumigatus control spore germination, vegetative growth, and the repression of additional secondary metabolites. This suggests that the dissection of the conserved Cand1-encoding gene within the genome of aspergilli was possible because it allowed the integration of a fungus-specific protein required for growth into the CandA complex in two different gene set versions, which might provide an advantage in evolution.IMPORTANCEAspergillus species are important for biotechnological applications, like the production of citric acid or antibacterial agents. Aspergilli can cause food contamination or invasive aspergillosis to immunocompromised humans or animals. Specific treatment is difficult due to limited drug targets and emerging resistances. The CandA complex regulates, as a receptor exchange factor, the activity and substrate variability of the ubiquitin labeling machinery for 26S proteasome-mediated protein degradation. Only Aspergillus species encode at least two proteins that form a CandA complex. This study shows that Aspergillus species had to integrate a third component into the CandA receptor exchange factor complex that is unique to aspergilli and required for vegetative growth, sexual reproduction, and activation of the ubiquitin labeling machinery. These features have interesting implications for the evolution of protein complexes and could make CandA-C1 an interesting candidate for target-specific drug design to control fungal growth without affecting the human ubiquitin-proteasome system.

Keywords: Aspergillus fumigatus; Aspergillus nidulans; COP9 signalosome; Cand1; Cullin-RING ubiquitin ligase; Nedd8; asexual development; protein complex; protein degradation; secondary metabolism; sexual development; spore germination.