Abstract
The COP9 (Constitutive photomorphogenesis 9) signalosome (CSN) is a highly conserved protein complex that influences several signaling and developmental processes. The COP9 signalosome consists of eight subunits, among which two subunits, CSN5 and CSN6, contain an Mpr1/Pad1 N-terminal (MPN) domain and the remaining six subunits contain a proteasome, COP9 signalosome, and initiation factor 3 (PCI) domain. In plants, each MPN subunit is encoded by two genes, which is not the case in other organisms. This review aims to provide in-depth knowledge of each COP9 signalosome subunit, concentrating on genetic analysis of both partial and complete loss-of-function mutants. At the beginning of this review, the role of COP9 signalosome in the hormonal signaling and defense is discussed, whereas later sections deal in detail with the available partial loss-of-function, hypomorphic mutants of each subunit. All available hypomorphic mutants are compared based on their growth response and deneddylation activity.
Highlights
The COP9 (Constitutive photomorphogenesis 9) signalosome (CSN) is an eight-subunit protein complex which is conserved throughout evolution
CSN5 and CSN6, possess an Mpr1/Pad1 N-terminal (MPN) domain and the remaining six contain a proteasome, COP9 signalosome, and initiation factor 3 (PCI) domain. Both MPN and PCI domains are present in lid subunit of 26S proteasome and eukaryotic translation initiation factor 3 [3]
Deneddylation activity or reduction of CSN subunits are not the only criteria to judge the severity of CSN mutant, considering that csn2-5 and virus-induced gene silencing (VIGS) of CSN5 show severe impairment in cullin neddylation in their respective subunits (50% in csn2-5 and 90% in VIGS of CSN5) but display mild dwarfism
Summary
The COP9 (Constitutive photomorphogenesis 9) signalosome (CSN) is an eight-subunit protein complex which is conserved throughout evolution. It was first identified in Arabidopsis as a repressor of light-regulated development [1,2]. CSN5 and CSN6, possess an Mpr1/Pad N-terminal (MPN) domain and the remaining six contain a proteasome, COP9 signalosome, and initiation factor 3 (PCI) domain. Viable CSN mutants with reduced activity demonstrate that CSN plays a major role in the growth and development of the plant. CSN regulates defense against pathogens by playing a crucial role in N gene-mediated resistance to tobacco mosaic virus [7], and in jasmonic acid-dependent plant defense responses [8]. The role of CSN in plants is limited to hormonal signaling and defense response and has a critical impact in many cellular, developmental processes [12] as well as in cell cycle progression [13]
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