Microtubule-dependent mRNA transport is coupled to endosomes and important for unipolar growth in Ustilago maydis
mRNA transport is an important mechanism to orchestrate polarised growth. Although basic principles of mRNA transport are well understood, key questions still remain open: Is mRNA transport connected to membrane trafficking and why do mRNAs move bidirectionally without accumulating subcellularly?...
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|mRNA transport is an important mechanism to orchestrate polarised growth. Although basic principles of mRNA transport are well understood, key questions still remain open: Is mRNA transport connected to membrane trafficking and why do mRNAs move bidirectionally without accumulating subcellularly?
The phytopathogenic basidiomycete Ustilago maydis grows in a highly polar manner. On the surface of its host plant maize fungal hyphae migrate via extended tip growth, which is a prerequisite for infection. Recently, the RNA recognition motif-harbouring protein Rrm4 was shown to be important for this asymmetric growth, since deletion of the corresponding gene causes an increased number of hyphae to establish two growth cones. Previous in vivo studies furthermore showed that Rrm4 binds RNAs and shuttles rapidly along microtubules.
In this study RNA live imaging revealed that Rrm4 mediates cytoplasmic shuttling of a set of mRNAs encoding for example the ubiquitin fusion protein Ubi1, the small G-protein Rho3 and septin Cdc3. However, the investigated mRNAs do not accumulate subcellularly. It was demonstrated that mRNA binding is essential for Rrm4 function and polar growth. Furthermore, the molecular motors kinesin-3 and dynein carry out antero- and retrograde transport of mRNAs. Intriguingly, the same motors that are mandatory for mRNA transport also mediate endosome trafficking.
Investigating septin mRNA transport applying dynamic live cell imaging revealed Rrm4-mediated co-transport of septin mRNA and encoded protein. Septins are highly conserved GTPases that function either in limiting membrane diffusion or as scaffold. Septins polymerise into higher-order structures and are involved in symmetry breaking and polar growth. Since co-localisation of mRNA and protein is indicative for local protein synthesis it was tested whether septin mRNA is directly translated on endosomes. Consistently, the accumulation of septin protein on endosomes is tightly linked to the recruitment of septin mRNA. Furthermore, ribosomal proteins co-localise with shuttling endosomes, but only in the presence of mRNA. Importantly, endosomal trafficking is essential for an efficient delivery of septin protein to septin filaments at growth cones.
In sum, the novel mechanism of mRNA-hitchhiking on endosomes that are trafficking due to the action of kinesin-3 and dynein was uncovered. This constitutes a novel mechanism for loading endosomes with septin protein by local translation - a process that might be important for rapid and coordinated assembly of septin filaments at hyphal growth cones.