Role of the MPN Subunits in COP9 Signalosome Assembly and Activity, and Their Regulatory Interaction with Arabidopsis Cullin3-Based E3 Ligases

摘要

The COP9 signalosome (CSN) is an evolutionarily conserved multisubunit protein complex that regulates a variety ofnbiological processes. Among its eight subunits, CSN5 and CSN6 contain a characteristic MPN (for Mpr1p and Pad1p Nterminal)ndomain and, in Arabidopsis thaliana, are each encoded by two genes: CSN5A, CSN5B and CSN6A, CSN6B,nrespectively. We characterized both MPN subunits using a series of single and double mutants within each gene family. Ournresults indicate that although CSN6A and CSN6B retain mostly redundant functions, CSN5A and CSN5B play unequal rolesnin the regulation of plant development. Complete depletion of either of the two MPN members results in CSN instability andnthe decay of various CSN components, along with the complete loss of CUL1, CUL3, and CUL4 derubylation. Furthermore,nwe demonstrate that CSN interacts with CUL3, in addition to CUL1 and CUL4, and that the lack of CSN activity differentiallynaffects the stability of those three cullins. Interestingly, we also show that optimal CUL3 activity is required to maintain thencellular pool of CSN5, through a posttranscriptional mechanism. Our data suggest the existence of reciprocal regulationnbetween CUL3 and CSN5 accumulation. This study thus completes the genetic analysis of all CSN subunits and confirms thenstructural interdependence between PCI and MPN subunits in functional CSN complex formation.