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Plant P-bodies (PBs) and stress granules (SGs)

In eukaryotic cells, gene expression is determined not only by transcription, but also by post-transcriptional events that control RNA processing, transport, stability, localization, and ultimately translation. Traditionally, it is thought that mRNAs are translated or degraded in the cytoplasm. However, when mRNAs fail to pass quality controls or translation is repressed by stress or miRNA, messenger ribonucleoprotein complexes (mRNPs) are temporarily stored in cytoplasmic foci known as Processing-bodies (P-bodies, PBs) or Stress Granules (SGs). Depending on the cellular condition, mRNAs in PBs or SGs can either re-engage with the normal translation cycle or enter the mRNA decay pathways for degradation. Hence PBs and SGs play pivotal roles in post-transcriptional regulation, as well as epigenetic modulation of gene expression [1, 2]. While PBs and SGs are distinct, they are compositionally and functionally linked. They work in concert as capacitors for mRNA metabolism under stress conditions. SGs are induced by many stresses such as heat, oxidative stress, UV irradiation, hypoxia, viral infection, and glucose starvation. PBs are often found in active growing cells and also induced by many stresses in mammalian cells. Interestingly, overexpression of a mammalian TZF, TTP, not only induces the assembly of PBs and SGs, but also promotes their physical interaction [3].

Although PBs and SGs have been studied intensively in mammals and yeast, it was unclear until recently if PBs and SGs exist in plants. In 2006-7, plant decapping complex proteins (DCP1, DCP2, and VCS/Hedls) were found in PB-like cytoplasmic foci by several research groups [4-6]. By contrast, a putative DEVH box RNA helicase was identified as the first plant SG component in the course of characterizing an Arabidopsis plasmodesmata mutant [7]. Later on, it was shown that mammalian counterpart PB components (DCP1, DCP2, and XRN4) and SG components (eIF4E, RBP47b, and UBP1b) were localized in cytoplasmic mRNP granules in plants [8]. These results indicate that plants appear to contain at least two distinct types of mRNP granules, but the distinction between PB and SG is still unclear. We have found that all 11 AtTZFs are localized in discrete cytoplasmic foci [9]. Since AtTZF1 co-localizes with putative PB and SG components, it raises a possibility that plant TZFs could localize to both PBs and SGs [10]. However, we have recently found that AtTZF1 does not co-localize with some putative PB or SG components, raising a possibility that mRNP granules in plants are deviated from mammals. Our current research is focused on the studies of composition, function, and assembly mechanism of plant P-bodies and stress granules.

AtTZF-1

   References

  1.  Anderson, P., and Kedersha, N. (2009). RNA granules: post-transcriptional and epigenetic modulators of gene expression. Nat Rev Mol Cell Biol 10, 430-436.
  2. Balagopal, V., and Parker, R. (2009). Polysomes, P bodies and stress granules: states and fates of eukaryotic mRNAs. Curr Opin Cell Biol 21, 403-408.
  3.  Kedersha, N., Stoecklin, G., Ayodele, M., Yacono, P., Lykke-Andersen, J., Fritzler, M.J., Scheuner, D., Kaufman, R.J., Golan, D.E., and Anderson, P. (2005). Stress granules and processing bodies are dynamically linked sites of mRNP remodeling. J Cell Biol 169, 871-884.
  4. Xu, J., Yang, J.Y., Niu, Q.W., and Chua, N.H. (2006). Arabidopsis DCP2, DCP1, and VARICOSE form a decapping complex required for postembryonic development. Plant Cell 18, 3386-3398.
  5. Goeres, D.C., Van Norman, J.M., Zhang, W., Fauver, N.A., Spencer, M.L., and Sieburth, L.E. (2007). Components of the Arabidopsis mRNA decapping complex are required for early seedling development. Plant Cell 19, 1549-1564.
  6.  Iwasaki, S., Takeda, A., Motose, H., and Watanabe, Y. (2007). Characterization of Arabidopsis decapping proteins AtDCP1 and AtDCP2, which are essential for post-embryonic development. FEBS Lett 581, 2455-2459.
  7. Kobayashi, K., Otegui, M.S., Krishnakumar, S., Mindrinos, M., and Zambryski, P. (2007). INCREASED SIZE EXCLUSION LIMIT 2 encodes a putative DEVH box RNA helicase involved in plasmodesmata function during Arabidopsis embryogenesis. Plant Cell 19, 1885-1897.
  8.  Weber, C., Nover, L., and Fauth, M. (2008). Plant stress granules and mRNA processing bodies are distinct from heat stress granules. Plant J.
  9. Pomeranz, M., Lin, P.C., Finer, J., and Jang, J.C. (2010). AtTZF gene family localizes to cytoplasmic foci. Plant Signal Behav 5, 190-192.
  10. 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.