title: Ubiquitin ligase switch in plant photomorphogenesis: A hypothesis.
abstract: The E3 ubiquitin ligase COP1 (CONSTITUTIVE PHOTOMORPHOGENIC1) plays a key role in the repression of the plant photomorphogenic development in darkness. In the presence of light, COP1 is inactivated by a mechanism which is not completely understood. This leads to accumulation of COP1's target transcription factors, which initiates photomorphogenesis, resulting in dramatic changes of the seedling's physiology. Here we use a mathematical model to explore the possible mechanism of COP1 modulation upon dark/light transition in Arabidopsis thaliana based upon data for two COP1 target proteins: HY5 and HFR1, which play critical roles in photomorphogenesis. The main reactions in our model are the inactivation of COP1 by a proposed photoreceptor-related inhibitor I and interactions between COP1 and a CUL4 (CULLIN4)-based ligase. For building and verification of the model, we used the available published and our new data on the kinetics of HY5 and HFR1 together with the data on COP1 abundance. HY5 has been shown to accumulate at a slower rate than HFR1. To describe the observed differences in the timecourses of the "slow" target HY5 and the "fast" target HFR1, we hypothesize a switch between the activities of COP1 and CUL4 ligases upon dark/light transition, with COP1 being active mostly in darkness and CUL4 in light. The model predicts a bi-phasic kinetics of COP1 activity upon the exposure of plants to light, with its restoration after the initial decline and the following slow depletion of the total COP1 content. CUL4 activity is predicted to increase in the presence of light. We propose that the ubiquitin ligase switch is important for the complex regulation of multiple transcription factors during plants development. In addition, this provides a new mechanism for sensing the duration of light period, which is important for seasonal changes in plant development.
authors.affiliation: School of Biological Sciences, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JH, United Kingdom. apokhilk@staffmail.ed.ac.uk
authors.forename: Alexandra
authors.initials: A
authors.lastname: Pokhilko
authors.forename: Jason A
authors.initials: JA
authors.lastname: Ramos
authors.forename: Hans
authors.initials: H
authors.lastname: Holtan
authors.forename: Don R
authors.initials: DR
authors.lastname: Maszle
authors.forename: Rajnish
authors.initials: R
authors.lastname: Khanna
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Millar
mesh_terms: D000465:Algorithms; D017360:Arabidopsis / Q000254:growth & development* / Q000378:metabolism / Q000528:radiation effects*; D029681:Arabidopsis Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D050976:Basic-Leucine Zipper Transcription Factors / Q000235:genetics / Q000378:metabolism; D003198:Computer Simulation; D044842:Cullin Proteins / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D015536:Down-Regulation / Q000528:radiation effects; D018506:Gene Expression Regulation, Plant / Q000502:physiology / Q000528:radiation effects; D007700:Kinetics; D008954:Models, Biological; D009024:Morphogenesis / Q000502:physiology / Q000528:radiation effects*; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism; D017440:Photoperiod; D044767:Ubiquitin-Protein Ligases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015854:Up-Regulation / Q000528:radiation effects
chemical_list: D029681:Arabidopsis Proteins; D050976:Basic-Leucine Zipper Transcription Factors; D044842:Cullin Proteins; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D044767:Ubiquitin-Protein Ligases; C514806:CULLIN4 protein, Arabidopsis; C414438:At1g02340 protein, Arabidopsis; C109254:HY5 protein, Arabidopsis; C471983:AT2G32950 protein, Arabidopsis