Transport of mRNAs is crucial to regulate protein expression at the spatio-temporal level. The localization of mRNA is coupled to the intracellular transport machinery by RNA-binding proteins (RBPs). A well-studied translocation mechanism is the endosomal mRNA transport in infectious hyphae of the plant pathogen Ustilago maydis. The key component of endosomal mRNA transport is the RBP Rrm4, which is crucial for the proper growth of fungal hyphae. Rrm4 contains three RNA Recognition Motifs (RRM) for RNA-protein interactions and three MLLE for protein-protein interactions. Based on iCLIP approach, it's known that Rrm4 binds predominantly to the 3'UTR of target transcripts and landmarks of translation (start and stop codons) which supports the current model of local translation on early endosomes. Furthermore, Rrm4 binds the sequence motif UAUG, which is present in one-third of all identified binding sites. UAUG occurs mostly in binding sites mapping to the Open Reading Frame (ORF) and start codons and is bound by the third RRM domain (RRM3). Mutations within RRM3, however, do not interfere with hyphal growth, whereas mutating the first RRM domain (RRM1 & RRM2) leads to a loss of function phenotype. This indicates that the RRM domains differentially contribute to RNA binding and may selectively affect distinct mRNA target sets. To further validate these findings and analyze the RRM domain phenotypically, behavior experiments under certain stress conditions were tested. The idea is to link the molecular phenotype of mutated RRM3 to biological phenotypes - which would contribute to the understanding of how multi-RRM proteins are working in general.

RNA-protein interaction in vivo: Endosomal mRNA transport in Ustilago maydis

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