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Targeted mRNA localization is a likely determinant of localized protein synthesis. To investigate whether mRNAs encoding mitochondrial proteins (mMPs) localize to mitochondria and, thus, might confer localized protein synthesis and import, we visualized endogenously expressed mMPs in vivo for the first time. We determined the localization of 24 yeast mMPs encoding proteins of the mitochondrial matrix, outer and inner membrane, and intermembrane space and found that many mMPs colocalize with mitochondria in vivo. This supports earlier cell fractionation and microarray-based studies that proposed mMP association with the mitochondrial fraction. Interestingly, a number of mMPs showed a dependency on the mitochondrial Puf3 RNA-binding protein, as well as nonessential proteins of the translocase of the outer membrane (TOM) complex import machinery, for normal colocalization with mitochondria. We examined the specific determinants of ATP2 and OXA1 mRNA localization and found a mutual dependency on the 3' UTR, Puf3, Tom7, and Tom70, but not Tom20, for localization. Tom6 may facilitate the localization of specific mRNAs as OXA1, but not ATP2, mRNA was mislocalized in tom6Δ cells. Interestingly, a substantial fraction of OXA1 and ATP2 RNA granules colocalized with the endoplasmic reticulum (ER) and a deletion in MDM10, which mediates mitochondria-ER tethering, resulted in a significant loss of OXA1 mRNA localization with ER. Finally, neither ATP2 nor OXA1 mRNA targeting was affected by a block in translation initiation, indicating that translation may not be essential for mRNA anchoring. Thus, endogenously expressed mRNAs are targeted to the mitochondria in vivo, and multiple factors contribute to mMP localization.

Localized mRNA translation is involved in cell-fate determination, polarization, and morphogenesis in eukaryotes. While various tools are available to examine mRNA localization, no easy and quick method has allowed for the visualization of endogenously expressed mRNAs in vivo. We describe a simple method (m-TAG) for PCR-based chromosomal gene tagging that uses homologous recombination to insert binding sites for the RNA-binding MS2 coat protein (MS2-CP) between the coding region and 3'-untranslated region of any yeast gene. Upon co-expression of MS2-CP fused with GFP, specific endogenously expressed mRNAs can be visualized in vivo for the first time. This method allows for the easy examination of mRNA localization using fluorescence microscopy and leaves the yeast cells amenable for further genetic analysis.

Targeted mRNA trafficking and local translation may play a significant role in controlling protein localization. Here we examined for the first time the localization of all ( approximately 50) mRNAs encoding peroxisomal proteins (mPPs) involved in peroxisome biogenesis and function. By using the bacteriophage MS2-CP RNA-binding protein (RBP) fused to multiple copies of GFP, we demonstrated that >40 endogenously expressed mPPs tagged with the MS2 aptamer form fluorescent RNA granules in vivo. The use of different RFP-tagged organellar markers revealed 3 basic patterns of mPP granule localization: to peroxisomes, to the endoplasmic reticulum (ER), and nonperoxisomal. Twelve mPPs (i.e., PEX1, PEX5, PEX8, PEX11-15, DCI1, NPY1, PCS60, and POX1) had a high percentage (52%-80%) of mRNA colocalization with peroxisomes. Thirteen mPPs (i.e., AAT2, PEX6, MDH3, PEX28, etc.) showed a low percentage (30%-42%) of colocalization, and 1 mPP (PEX3) preferentially localized to the ER. The mPPs of the nonperoxisomal pattern (i.e., GPD1, PCD1, PEX7) showed <<30% colocalization. mPP association with the peroxisome or ER was verified using cell fractionation and RT-PCR analysis. A model mPP, PEX14 mRNA, was found to be in close association with peroxisomes throughout the cell cycle, with its localization depending in part on the 3'-UTR, initiation of translation, and the Puf5 RBP. The different patterns of mPP localization observed suggest that multiple mechanisms involved in mRNA localization and translation may play roles in the importation of protein into peroxisomes.

This protocol describes m-TAG, a novel method for the visualization of endogenously expressed mRNAs in live yeast (Saccharomyces cerevisiae). First, a gene of interest is tagged with multiple binding sites for the RNA-binding MS2 coat protein (MS2-CP), using a PCR-based genomic-tagging strategy and homologous recombination. Next, MS2-CP fused to GFP(x3) is expressed in cells; binding of this fusion protein to the tagged mRNA yields an RNA granule that can be visualized by fluorescence microscopy. While existing methods necessitate cell fixation (for in situ hybridization) or the detection of exogenously expressed mRNAs (from plasmids), or give transient signals (i.e., with fluorescent hybridization probes), m-TAG allows for the robust and stable visualization of endogenously expressed mRNAs in vivo and facilitates the study of mRNA dynamics under different growth conditions. The m-TAG procedure is simple, easy to perform and takes <3 weeks to yield cultured yeast strains for mRNA visualization.