The RNA-binding protein Rrm4 is essential for polarity in Ustilago maydis and shuttles along microtubules.
Formation of polar-growing hyphae is essential for infection by the plant pathogen Ustilago maydis. Here we observe that loss of RNA-recognition motif protein Rrm4 caused formation of abnormal hyphae. The insertion of septa at the distal pole was abolished and a significantly increased number of hyphae grew bipolarly. UV-crosslinking experiments revealed that Rrm4 bound RNA via its N-terminal RRMs and that its RNA-binding activity was substantially increased during filamentation. Rrm4 assembled into particles that shuttled bidirectionally along microtubules to both poles. Recruitment of Rrm4 into particles increased during filamentation, and mutations in the peptide-binding pocket of its PABC domain caused abnormal particle formation as well as polarity defects. Shuttling was mediated by active transport because loss of conventional kinesin, which interferes with the balance of microtubule-dependent motors, caused accumulation of particles at the poles resulting in disturbed polarity. Thus, constant transport of the RNA-binding protein towards the poles is needed to orchestrate hyphal growth. Since a mutation of the N-terminal RRM that leads to reduced RNA binding in vivo also affected polarity, Rrm4 might regulate polarity of the infectious hyphae by transporting RNA from the nucleus to cell poles.
Becht
Philip
P
Max Planck Institute for Terrestrial Microbiology, Department for Organismic Interactions, Karl-von-Frisch-Str., 35043 Marburg, Germany.
König
Julian
J
Feldbrügge
Michael
M
D000595:Amino Acid Sequence; D016764:Cell Polarity / Q000502:physiology*; D003599:Cytoskeleton / Q000378:metabolism; D005656:Fungal Proteins / Q000378:metabolism / Q000502:physiology; D016547:Kinesins / Q000502:physiology; D008870:Microtubules / Q000378:metabolism*; D008969:Molecular Sequence Data; D030781:Organisms, Genetically Modified; D021381:Protein Transport; D034443:RNA Transport / Q000502:physiology; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D017386:Sequence Homology, Amino Acid; D014588:Ustilago / Q000235:genetics / Q000254:growth & development / Q000378:metabolism* / Q000502:physiology*
D005656:Fungal Proteins; C107102:KIN1 protein, Ustilago maydis; D016601:RNA-Binding Proteins; D016547:Kinesins