Updated project metadata. The eight-subunit COP9 signalosome (CSN complex), controls the exchange of E3 ubiquitin cullin RING ligase receptors through its deneddylase activity, providing specificity to eukaryotic protein degradation. The conserved eight-subunit CSN complex is required for multicellular development of the filamentous fungus Aspergillus nidulans. The human, as well as the fungal CSN complex assembles through a heptameric subcomplex (pre-CSN complex) that is activated by the integration of the eighth subunit, the catalytic CsnE/5 deneddylase. The focus of this work was to unveil the assembly of the native fungal pre-CSN complex via combined genetic and biochemical approaches. Interactomes of various functional GFP-Csn subunit fusions, produced in pre-CSN deficient fungal double csn subunit gene deletion strains, were compared by affinity purifications coupled to mass spectrometry analyses. With this approach, two distinct heterotrimeric CSN subcomplexes were identified as pre-CSN assembly intermediates, namely CsnA/1-C/3-H/8 and CsnD/4-F/6-G/7 that form independently of CsnB/2 that connects the heterotrimers to a heptameric subcomplex and enables subsequent integration of CsnE/5, yielding in the enzymatically active CSN holocomplex. Surveillance mechanisms control accurate Csn subunit abundances and correct cellular localization for sequential assembly, since deprivation of Csn subunits change the abundance and location of the remaining Csn subunits.