Targeted degradation of TOC1 by ZTL modulates circadian function in Arabidopsis thaliana

Abstract

The underlying mechanism of circadian rhythmicity appears to be conserved among organisms, and is based on negative transcriptional feedback loops forming a cellular oscillator (or 'clock'). Circadian changes in protein stability, phosphorylation and subcellular localization also contribute to the generation and maintenance of this clock. In plants, several genes have been shown to be closely associated with the circadian system. However, the molecular mechanisms proposed to regulate the plant clock are mostly based on regulation at the transcriptional level. Here we provide genetic and molecular evidence for a role of ZEITLUPE (ZTL) in the targeted degradation of TIMING OF CAB EXPRESSION 1 (TOC1) in Arabidopsis thaliana (thale cress). The physical interaction of TOC1 with ZTL is abolished by the ztl-1 mutation, resulting in constitutive levels of TOC1 protein expression. The dark-dependent degradation of TOC1 protein requires functional ZTL, and is prevented by inhibiting the proteosome pathway. Our results show that the TOC1-ZTL interaction is important in the control of TOC1 protein stability, and is probably responsible for the regulation of circadian period by the clock.

PSEUDO RESPONSE REGULATORs stabilize CONSTANS protein to promote flowering in response to day length

“…PRR proteins accumulate during the day and are degraded during the night (Mas et al , 2003b; Farré & Kay, 2007; Ito et al , 2007; Kiba et al , 2007; Fujiwara et al , 2008), so they could directly contribute to CO protein stabilization in light. CO is also known to be stabilized in plants exposed to blue light (BL) and far‐red light (FR) (Valverde et al , 2004).…”

“…Overexpression of PRR5 slightly affects the peak time of some PRRs under LDs, but mutations in PRR genes do not affect the peak time of expression of other PRR s under these conditions (Appendix Fig S4A and B). These proteins are degraded during the night, so they mainly accumulate during the day when CO protein also accumulates (Mas et al , 2003b; Farré & Kay, 2007; Ito et al , 2007; Kiba et al , 2007). Through this regulation, PRRs allow detection of long‐light periods and contribute to recognition of LDs, interacting with and stabilizing CO protein during the day specifically under LDs.…”

“…The pattern of accumulation of PRR proteins in the light is in part mediated by their degradation in the dark. TOC1 and PRR5 proteins are targeted for degradation by ZTL, which was originally identified as a component of the circadian clock (Somers et al , 2000; Mas et al , 2003b; Kiba et al , 2007). It is still not known how PRR7 and PRR9 proteins are degraded during the night.…”

“…The abundance of their transcripts and proteins exhibit circadian rhythms, and under diurnal conditions, they peak in expression sequentially at 2–3 h intervals during the light period in the order PRR9 , PRR7 , PRR5 , PRR3 , and TOC1 (Matsushika et al , 2000; Fujiwara et al , 2008). PRR9, PRR7, PRR5, and TOC1 proteins are degraded during the night, so they mainly accumulate during the day when they repress transcription of genes encoding other clock components such as LATE ELONGATED HYPOCOTYL (LHY) and CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) (Mas et al , 2003b; Farré & Kay, 2007; Ito et al , 2007; Kiba et al , 2007; Nakamichi et al , 2010; Huang et al , 2012). Mutations in the PRR genes also delay flowering under LDs (Nakamichi et al , 2005; Ito et al , 2008), but double or triple mutants exhibit much stronger phenotypes suggesting functional redundancy between the genes (Nakamichi et al , 2005, p. 549; Ito et al , 2008).…”

Arabidopsis CRY2 and ZTL mediate blue-light regulation of the transcription factor CIB1 by distinct mechanisms

“…FKF1 mediates blue-light-dependent degradation of CDF1 and stabilization of the CO protein to facilitate transcription of FT (15,16). ZTL acts as the substratebinding subunit of the SCF ZTL E3 ubiquitin ligase that regulates protein abundance of the key circadian oscillator components, TIMING of CAB EXPRESSION 1 (TOC1) and PSEUDO-RESPONSE REGULATOR 5 (PRR5), to affect expression of a number of flowering-time genes, including FT (16)(17)(18).…”