Characterization of the last subunit of the Arabidopsis COP9 signalosome

Integration of multiple ubiquitin signals in proteasome regulation

The ubiquitin-proteasome system has emerged in the last decades as a new paradigm in cell physiology. Ubiquitin is found in fundamental levels of cell regulation, as a target for degradation to the proteasome or as a signal that controls protein function in a complex manner. Even though many aspects of the ubiquitin system remain unexplored, the contributions on the field uncover that ubiquitin represents one of the most sophisticated codes in cellular biology. The proteasome is an ATP-dependent protease that degrades a large number of protein substrates in the cell. The proteasome recruits substrates by a number of receptors that interact with polyubiquitin. Recently, it has been shown that one of these receptors, Rpn10, is regulated by monoubiquitination. In this chapter, we show an overview of the central aspects of the pathway and describe the methodology to characterize in vitro the monoubiquitination of proteasome subunits.

Role of the MPN subunits in COP9 signalosome assembly and activity, and their regulatory interaction with Arabidopsis Cullin3-based E3 ligases

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

Proteasomes from Structure to Function: Perspectives from Archaea

Insight into the world of proteolysis has expanded considerably over the past decade. Energy-dependent proteases, such as the proteasome, are no longer viewed as nonspecific degradative enzymes associated solely with protein catabolism but are intimately involved in controlling biological processes that span life to death. The proteasome maintains this exquisite control by catalyzing the precisely timed and rapid turnover of key regulatory proteins. Proteasomes also interplay with chaperones to ensure protein quality and to readjust the composition of the proteome following stress. Archaea encode proteasomes that are highly related to those of eukaryotes in basic structure and function. Investigations of archaeal proteasomes coupled with those of eukaryotes has greatly facilitated our understanding of the molecular mechanisms that govern regulated protein degradation by this elaborate nanocompartmentalized machine.