Integration of the catalytic subunit activates deneddylase activity in vivo as final step in fungal COP9 signalosome assembly.

The eight-subunit COP9 signalosome (CSN) is conserved from filamentous fungi to humans and functions at the interface between cellular signalling and protein half-life control. CSN consists of six PCI and two MPN domain proteins and forms a scaffold for additional interacting proteins. CSN controls protein stability in the ubiquitin-proteasome system where the MPN domain CSN5/CsnE subunit inactivates cullin-RING ligases. The CSN5/CsnE isopeptidase functions as deneddylase and removes the ubiquitin-like protein Nedd8. The six PCI domain proteins of human CSN form a horseshoe-like ring and all eight subunits are connected by a bundle of C-terminal α-helices. We show that single deletions of any csn subunit of Aspergillus nidulans resulted in the lack of deneddylase activity and identical defects in the coordination of development and secondary metabolism. The CSN1/CsnA N-terminus is dispensable for deneddylase activity but required for asexual spore formation. Complex analyses in mutant strains revealed the presence of a seven-subunit pre-CSN without catalytic activity. Reconstitution experiments with crude extracts of deletion strains and recombinant proteins allowed the integration of CSN5/CsnE into pre-CSN resulting in an active deneddylase. This supports a stable seven subunit pre-CSN intermediate where deneddylase activation in vivo can be controlled by CSN5/CsnE integration as final assembly step.

An eight-subunit COP9 signalosome with an intact JAMM motif is required for fungal fruit body formation.

Fruit body formation in filamentous fungi is a complex and yet hardly understood process. We show here that protein turnover control is crucial for Aspergillus nidulans development. Deletion of genes encoding COP9 signalosome (CSN) subunits 1, 2, 4, or 5 resulted in identical blocks in fruit body formation. The CSN multiprotein complex controls ubiquitin-dependent protein degradation in eukaryotes. Six CSN subunits interacted in a yeast two-hybrid analysis, and the complete eight-subunit CSN was recruited by a functional tandem affinity purification tag fusion of subunit 5 (CsnE). The tagged CsnE was unable to recruit any CSN subunit in a strain deleted for subunit 1 or subunit 4. Mutations in the JAMM metalloprotease core of CsnE resulted in mutant phenotypes identical to those of csn deletion strains. We propose that a correctly assembled CSN including a functional JAMM links protein turnover to fungal sexual development.

Regulation of the acuF gene, encoding phosphoenolpyruvate carboxykinase in the filamentous fungus Aspergillus nidulans.

Phosphoenolpyruvate carboxykinase (PEPCK) is a key enzyme required for gluconeogenesis when microorganisms grow on carbon sources metabolized via the tricarboxylic acid (TCA) cycle. Aspergillus nidulans acuF mutants isolated by their inability to use acetate as a carbon source specifically lack PEPCK. The acuF gene has been cloned and shown to encode a protein with high similarity to PEPCK from bacteria, plants, and fungi. The regulation of acuF expression has been studied by Northern blotting and by the construction of lacZ fusion reporters. Induction by acetate is abolished in mutants unable to metabolize acetate via the TCA cycle, and induction by amino acids metabolized via 2-oxoglutarate is lost in mutants unable to form 2-oxoglutarate. Induction by acetate and proline is not additive, consistent with a single mechanism of induction. Malate and succinate result in induction, and it is proposed that PEPCK is controlled by a novel mechanism of induction by a TCA cycle intermediate or derivative, thereby allowing gluconeogenesis to occur during growth on any carbon source metabolized via the TCA cycle. It has been shown that the facB gene, which mediates acetate induction of enzymes specifically required for acetate utilization, is not directly involved in PEPCK induction. This is in contrast to Saccharomyces cerevisiae, where Cat8p and Sip4p, homologs of FacB, regulate PEPCK as well as the expression of other genes necessary for growth on nonfermentable carbon sources in response to the carbon source present. This difference in the control of gluconeogenesis reflects the ability of A. nidulans and other filamentous fungi to use a wide variety of carbon sources in comparison with S. cerevisiae. The acuF gene was also found to be subject to activation by the CCAAT binding protein AnCF, a protein homologous to the S. cerevisiae Hap complex and the mammalian NFY complex.