Structural insights into the function of the core-circadian factor TIMING OF CAB2 EXPRESSION 1 (TOC1)

To extend further our understanding of the TOC1 protein, we performed a ROSETTA structural prediction on TOC1 orthologues from four plant species. Phylogenetic interpretations assisted in model construction.From our models, we suggest that TOC1 is a three-domain protein: TOC1 has an amino-terminal signaling-domain related to response receivers, a carboxy-terminal domain that could participate both in metal binding and in transcriptional regulation, and a linker domain that connects the two.The models we present should prove useful in future hypothesis-driven biochemical analyses to test the predictions that TOC1 is a multi-domain signaling component of the plant circadian clock.Circadian clocks are prevalent timing mechanisms used to predict the daily changes present in the 24-h day-night cycle. In plants, this clock regulates several developmental and metabolic processes. Dominant outputs include the oscillation of free-cytosolic calcium (Ca2+) [1], which are generated from cADPR-derived signals [2], and the rhythmic accumulation of around 10% of all transcripts [2-6]. In particular, transcription factors are over-represented as cycling gene products [3,7]. In this way, the circadian timer drives numerous molecular outputs in the establishment of fitness in physiological processes and developmental timing. This fitness benefit has been confirmed [8]. The current aims on studies of the mechanism of the plant clock are to define the factors that contribute to rhythm-generating properties of the oscillator.Molecular-genetic analyses have lead to a framework understanding of the core elements that make up the circadian clock. Mutants of Arabidopsis thaliana that are clock defective have been used to identify loci critical for normal rhythmicity. TIMING OF CAB2 EXPRESSION 1 (TOC1) was the first such locus identified [9], and TOC1 continues to be placed central within the clock mechanism [10-14]. Extending from these studies, many clock genes are reciprocally regulated,