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Plant Tandem CCCH Zinc Finger (TZF) Proteins

Sugar sensing and signal transduction is pivotal for coordinating growth and stress responses in plants. Our transcriptome analysis has uncovered 185 sugar responsive nuclear transcription factors in Arabidopsis thaliana [1]. Unexpectedly, one of the transcription factors, Tandem CCCH Zinc Finger1 (AtTZF1), can traffic between nucleus and cytoplasmic foci. These foci are characteristic of recently discovered mammalian Processing-bodies (P-bodies or PBs) and Stress Granules (SGs). PBs and SGs are cytoplasmic messenger ribonucleoprotein (mRNP) complex, playing important roles in post-transcriptional regulation and epigenetic modulation of gene expression. While PBs are enriched with translation-repressed mRNAs and mRNA silencing and degradation machineries, SGs are super-complexes of stalled pre-initiation complexes and translation regulators. Notably, AtTZF1-associated PBs and SGs are assembled dynamically in specific plant tissues under particular stresses. They are preferentially formed in stomata and stem cells, and can be induced by stress hormone methyl jasmonate [2]. Reverse genetic analyses indicate that AtTZF1 acts as a positive regulator of sugar and ABA (stress) response, and a negative regulator of GA (growth) response. Plants over-expressing AtTZF1 are compact, late flowering, and superior in cold- and drought-tolerance (Figure 1). In addition, ABA-, Salicylic Acid- (SA-), and sugar-responsive genes are up regulated, whereas GA-responsive genes are down regulated in AtTZF1 OX plants [3]. Consistent with these findings, numerous reports have shown recently that plant TZFs are powerful switches for growth, stress, and gene expression responses [4, 5].

gene expression

In Arabidopsis, all 11 TZF subfamily IX proteins (AtTZFs) contain a unique TZF motif variant preceded by an arginine-rich motif (RR-TZF) that is distinct from the mammalian TZF motif. The mammalian prototypic TZF- called TTP, can induce mRNA degradation by recruiting RNA degradation machineries [6] and binding to the AU Rich Elements (AREs) at the 3’UTR of many pivotal regulators, including Tumor Necrosis Factor α (TNF-α). Although AtTZF1 can bind both DNA and RNA in vitro [2], it does not bind the consensus AREs with high affinity, and both RR and TZF domains are required for RNA binding. However, RNA half-life analysis indicates that AtTZF1 can trigger degradation of ARE-containing mRNAs [7]. On the basis of these results, we hypothesize that AtTZF1 is likely an RNA regulator. Work is in progress to reveal: 1) The AtTZF1 in vivo RNA target sites using enhanced UV crosslinking and immunoprecipitation (eCLIP) [8] and infra-red CLIP (irCLIP) [9] of ribonucleoprotein complexes, 2) Role of AtTZF1 in RNA regulation, 3) Mechanisms that regulate AtTZF1 intracellular trafficking, 4) Role of AtTZF1 in PB and SG assembly and function, 5) The components of AtTZF1 associated mRNP complex.
 

References

  1. Price, J., Laxmi, A., St Martin, S.K., and Jang, J.C. (2004). Global transcription profiling reveals multiple sugar signal transduction mechanisms in Arabidopsis. Plant Cell 16, 2128-2150.
  2. Pomeranz, M.C., Hah, C., Lin, P.C., Kang, S.G., Finer, J.J., Blackshear, P.J., and Jang, J.C. (2010). The Arabidopsis tandem zinc finger protein AtTZF1 traffics between the nucleus and cytoplasmic foci and binds both DNA and RNA. Plant Physiol 152, 151-165.
  3.  Lin, P.C., Pomeranz, M.C., Jikumaru, Y., Kang, S.G., Hah, C., Fujioka, S., Kamiya, Y., and Jang, J.C. (2011). The Arabidopsis tandem zinc finger protein AtTZF1 affects ABA- and GA-mediated growth, stress and gene expression responses. Plant J 65, 253-268.
  4. Bogamuwa, S.P., and Jang, J.C. (2014). Tandem CCCH zinc finger proteins in plant growth, development and stress response. Plant Cell Physiol 55, 1367-1375.
  5. Jang, J.C. (2016). Arginine-rich motif-tandem CCCH zinc finger proteins in plant stress responses and post-transcriptional regulation of gene expression. Plant Science 252, 118-124.
  6.  Lykke-Andersen, J., and Wagner, E. (2005). Recruitment and activation of mRNA decay enzymes by two ARE-mediated decay activation domains in the proteins TTP and BRF-1. Genes Dev 19, 351-361.
  7. Qu, J., Kang, S.G., Wang, W., Musier-Forsyth, K., and Jang, J.C. (2014). The Arabidopsis thaliana tandem zinc finger 1 (AtTZF1) protein in RNA binding and decay. Plant J 78, 452-467.
  8.  Van Nostrand, E.L., Pratt, G.A., Shishkin, A.A., Gelboin-Burkhart, C., Fang, M.Y., Sundararaman, B., Blue, S.M., Nguyen, T.B., Surka, C., Elkins, K., et al. (2016). Robust transcriptome-wide discovery of RNA-binding protein binding sites with enhanced CLIP (eCLIP). Nat Methods 13, 508-514.
  9.  Zarnegar, B.J., Flynn, R.A., Shen, Y., Do, B.T., Chang, H.Y., and Khavari, P.A. (2016). irCLIP platform for efficient characterization of protein-RNA interactions. Nat Methods 13, 489-492.