A role of Arabidopsis COP9 signalosome in multifaceted developmental processes revealed by the characterization of its subunit 3

The COP9 signalosome is a highly conserved eight-subunit protein complex in itially defined as a repressor of photomorphogenic development in Arabidops is. It has recently been suggested that the COP9 signalosome directly inter acts and regulates SCF type E3 ligases, implying a key role in ubiquitin-pr oteasome mediated protein degradation. We report that Arabidopsis FUS11 gen e encodes the subunit 3 of the COP9 signalosome (CSN3). The fus11 mutant is defective in the COP9 signalosome and accumulates significant amount of mu lti-ubiquitinated proteins. The same mutant is specifically impaired in the 26S proteasome-mediated degradation of HY5 but not PHYA, indicating a sele ctive involvement in protein degradation. Reduction-of-function transgenic lines of CSN3 produced through gene co-suppression also accumulate multi-ub iquitinated proteins and exhibit diverse developmental defects. This result substantiates a hypothesis that the COP9 signalosome is involved in multif aceted developmental processes through regulating proteasome-mediated prote in degradation.