The timing of many physiological and developmental processes in most eukaryotes is under the control of a circadian clock. This endogenous, self-sustaining oscillator maintains a rhythm of ca. 24 h in processes as diverse as human sleep/wake cycles, insect pupal eclosion, fungal sporulation and the movement of plant leaves. Many of the key events in plant development, such as flowering time, depend on receiving the appropriate environmental signals at the right time, and the circadian timekeeping mechanism allows them to keep pace with and anticipate cyclic events in their environment.
The central pacemaker driving circadian rhythms in plants consists of one or more autoregulatory feedback loops that are still being molecularly dissected. Work in my lab focuses on post-transcriptional control of the clock and in defining the molecular components that comprise the oscillator.
Post-translational regulation: proteostasis
ZEITLUPE (ZTL) is an F-box protein originally isolated as a long period mutant in Arabidopsis. It, and two related family members, are unique among the nearly 700 plant F-box proteins in possessing a blue-light sensing LOV (Light Oxygen and Voltage) domain at its N-terminus, very similar to the flavin-binding regions found in the phototropins. ZTL targets two members of a closely related family of pseudoresponse regulators (TOC1 and PRR5) that are key in setting the pace of the oscillator. Our work with ZTL has focused on understanding the role of each of three domains that comprise the protein, in the context of the circadian system. We identified ZTL as a novel blue light photoreceptor, the first F-box protein to possess this property (Kim et al., 2007).
Interactors with ZTL that regulates its stability in vivo are HSP90 and GIGANTEA (GI). GI has a wide-ranging role throughout plant development, including control over circadian period. GI confers post-translational control of circadian cycling to ZTL protein, acting as a co-chaperone with HSP90 to facilitate ZTL maturation. This finding identified a molecular role for GI and uncovered a new post-translational mechanism for establishing a circadian rhythm in eukaryotes (Cha et al., 2017).
Post-translational regulation: phosphorylation
Role of protein phosphorylation in the control of period and robustness in the circadian clock. Five pseudoresponse regulator (PRR) proteins (TOC1, PRR3, PRR5, PRR7 and PRR9) are sequentially expressed over a 24 h diurnal and circadian cycle, and elimination and overexpression of each affects the period or robustness of the circadian oscillator. The phosphorylation state of most of them is also circadian regulated, and interaction between TOC1 and PRR3 depends on the phosphorylation state of both proteins (Fujiwara et al., 2008). The phosphorylation-based interaction between TOC1 and PRR3 competitively inhibits the TOC1/ZTL interaction, protecting TOC1 from SCFZTL-dependent degradation and altering circadian amplitude. In contrast, a TOC1/PRR5 interaction affects TOC1 phosphorylation, nuclear import and subnuclear localization (Wang et al., 2010). We are interested in understanding the role of the phosphorylation states of these proteins in regulating their interactions, and in their control of circadian period.