Multicellular organisms are organized into different tissues with specialized functions. Similarly, cells spatially organize their intracellular content into distinct regions with unique functions. Intracellular organization occurs at many levels, for example both organelles and individual proteins localize to specific subcellular sites. In addition, many mRNAs are targeted to specific locations as well. mRNA localization is essential in developing organisms, as is clear from work in Drosophila and Xenopus embryos, where mRNA localization is thought to play a pivotal role in cell fate determination. In adult neurons many mRNAs are known to localize to specific sites as well, allowing spatial control of protein synthesis. While mRNA localization was originally thought to be limited to specific cases, it is now becoming clear that mRNA localization may be a very general aspect of mRNA biology.
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We have studied mRNA localization in the unicellular budding yeast S. cerevisiae and found that the mRNA encoding the Ash1 protein, a cell fate determinant, localizes specifically to the tip of the growing bud (Takizawa et al., 1997). Further analysis revealed that the myosin motor protein Myo4 binds the Ash1 mRNA through several adaptor proteins, including She2/3, and transports it to the bud tip along actin cables (Takizawa et al., 1997; Takizawa and Vale, 2000). This transport is not only essential for Ash1 mRNA localization, but also to control the fate of the daughter cell. Using microarray experiments on immuno-precipitations of the proteins involved in mRNA transport, we subsequently identified a large group of mRNAs that are specifically transported to the bud tip (Takizawa et al., 2000; Shepard et al., 2003). Interestingly, one of these mRNAs, called IST2, is initially transported to the bud tip by Myo4, but is subsequently maintained in the bud by a septin-based diffusion barrier (Takizawa et al., 2000), indicating that bud localization of mRNAs can be a complex and multi-step process.
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To get at the biochemical nature of mRNA transport complexes, we also studied mRNA localization in Drosophila embryos. In work by Jim Wilhelm (Wilhelm et al., 2000), we identified a large complex that is involved in localizing Oskar mRNA to the posterior pole of the embryo. This complex contains a know protein (Exuperantia) but also several other proteins, some of which have been subsequently studied by Jim Wilhelm and his lab at UCSD (see Jim Wilhelm’s web site).
After not working on RNA localization for many years, we are beginning to work on it again. Current efforts focus on studies in mammalian cells for which we are trying to develop new tools to study the localization and dynamics of different mRNA molecules in living cells.
See additional movies and images here.
- (pdf) - Shepard, K.A., Gerber, A.P., Jambhekar,
A., Takizawa, P.A., Brown, P.O.,
Herschlag, D., DeRisi,
J.L. and Vale, R.D.. (2003) Widespread cytoplasmic mRNA transport in
yeast: Identification of 22 bud-localized transcripts using DNA microarray
analysis. Proc Natl Acad Sci USA 100: 11429-11434.
- (pdf)- Takizawa, P.A., DeRisi,
J.L., Wilhelm, J. E., and Vale, R. D. (2000) Plasma
membrane compartmentalization in yeast by messenger RNA transport and a
septin diffusion barrier. Science 290: 341-344.
- (pdf) - Takizawa, P.A. and Vale, R.D.
(2000) The myosin motor, Myo4p, binds Ash1 mRNA via the adapter protein,
She3p. Proc Natl Acad Sci USA 97: 5273-5278.
- (pdf) - Wilhelm, J. E., Mansfield, J., Hom-Booher, N., Wang, S., Turck,
C., Hazelrigg, T. and Vale, R. D. (2000) Isolation of a ribonucleoprotein
complex involved in mRNA localization in Drosophila oocytes. J Cell Biol 148: 427-439.
- (pdf) - Takizawa, Peter A., Sil, Anita,
Swedlow, Jason R., Herskowitz, Ira and Vale, Ronald D. (1997) Actin-dependent
localization of an RNA encoding a cell-fate determinant in yeast. Nature