Browsing by Author "Huang, He"

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    The eta7/csn3-3 auxin response mutant of Arabidopsis defines a novel function for the CSN3 subunit of the COP9 signalosome.
    (2012-07) Huang, He
    The COP9 signalosome (CSN) is an eight subunit protein complex conserved in all higher eukaryotes. In Arabidopsis thaliana, the CSN regulates plant auxin response by removing the ubiquitin-like protein NEDD8/RUB1 from the CUL1 subunit of the SCFTIR1/AFB ubiquitin-ligase (deneddylation). Previously described null mutations in any CSN subunit resulted in the pleiotropic cop/det/fus phenotype and caused seedling lethality, hampering the study of CSN functions in plant development. In a genetic screen to identify enhancers of the auxin response defects conferred by the tir1-1 mutation, we identified a viable csn mutant of subunit 3 (CSN3), designated eta7/csn3-3. In comparison with eta6/csn1-10, which was identified in the same enhancer screen (Zhang et al., 2008), both csn3-3 and csn1-10 enhanced the auxin response defects of tir1-1. Similar to csn1-10, csn3-3 also confers several phenotypes associated with impaired auxin signaling, including auxin resistant root growth and diminished auxin responsive gene expression. Surprisingly however, unlike csn1-10 as well as other previously characterized csn mutants, csn3-3 plants are not defective in either the CSN-mediated deneddylation of CUL1 or in SCFTIR1/AFB mediated degradation of Aux/IAA proteins. These findings suggest that csn3-3 is an atypical csn mutant that defines a novel CSN or CSN3-specific function. Consistent with this possibility, I observed dramatic differences in double mutant interactions between csn3-3 and other auxin signaling mutants compared to csn1-10. Lastly, unlike other csn mutants, assembly of the CSN holocomplex was unaffected in csn3-3 plants. However, I detected a small CSN3-containing protein complex (sCSN3c) that was altered in csn3-3 plants. I hypothesize that in addition to its role in the CSN as a cullin deneddylase, CSN3 functions in a smaller protein complex that is required for proper auxin signaling. Analyses on the purification of sCSN3c suggested that it is not likely a dimer of CSN3, or a CSN subcomplex. My data resulting from sCSN3c purification using various chromatographic steps provide useful information necessary for identifying the components of the complex.