Polar growth of filaments of Ustilago maydis depends on long-distance transport of mRNAs. The key RNA-binding protein Rrm4 transports distinct sets of mRNAs as part of ribonucleoprotein (mRNP) complexes. Corresponding mRNAs encode polarity factors e.g. the small G protein Rho3 and the septin Cdc3 as well as mitochondrial proteins. Polar accumulation of Cdc3 at hyphal tips is disturbed in rrm4Δ strains, indicating that transport of target mRNAs determines the correct subcellular localisation of encoded proteins. Although the RNA-binding protein and its cargo mRNAs are known, the exact mechanism(s) of transport remains elusive. Here, we aim to investigate the protein composition of Rrm4-containing mRNPs to further elucidate the mechanism(s) of mRNA trafficking. Since Rrm4 is an integral part of transport mRNPs, it will be used as a molecular handle to purify associated components. Interesting candidates will be further characterised to uncover their function e.g. during RNA binding as well as during formation and movement of mRNP complexes. These studies will allow the identification and functional classification of novel trafficking components and provide deeper insights into the biological processes underlying polar growth in eukaryotes.
Microtubule-dependent shuttling of Rrm4 in filaments of U. maydis. (A) b-dependent filaments carrying a wildtype allele (rrm4) or a deletion allele (rrm4Δ). Retraction septa and polar growing apex are marked by arrowheads and asterisks, respectively (Zarnack & Feldbrügge, 2007). (B) In vivo co-localisation of Rrm4R (Rrm4 fused to mRfp; red, arrows) with dynamic Tub1G-decorated microtubules (Tub1 fused to eGfp; green) in AB33rrm4R /tub1G. Rectangles indicate regions that are magnified below (scale bars, 10 µm; elapsed time is given in seconds).