The COP9 signalosome regulates cell proliferation of Dictyostelium discoideum.

Regulated protein destruction involving SCF (Skp1/Cullin/F-box, E3 ubiquitin ligase) complexes is required for multicellular development of Dictyostelium discoideum. Dynamic modification of cullin by nedd8 is required for the proper action of SCF. The COP9 signalosome (CSN), first identified in a signaling pathway for light response in plants, functions as a large multi-protein complex that regulates cullin neddylation in eukaryotes. Still, there is extreme sequence divergence of CSN subunits of the yeasts in comparison to the multicellular plants and animals. Using the yeast two-hybrid system, we have identified the CSN5 subunit as a potential interacting partner of a cell surface receptor of Dictyostelium. We further identified and characterized all 8 CSN subunits in Dictyostelium discoideum. Remarkably, despite the ancient origin of Dictyostelium, its CSN proteins cluster very closely with their plant and animal counterparts. We additionally show that the Dictyostelium subunits, like those of other systems are capable of multi-protein interactions within the CSN complex. Our data also indicate that CSN5 (and CSN2) are essential for cell proliferation in Dictyostelium, a phenotype similar to that of multicellular organisms, but distinct from that of the yeasts. Finally, we speculate on a potential role of CSN in cullin function and regulated protein destruction during multicellular development of Dictyostelium.