Target mRNAs and binding sites of the key RNA-binding protein Rrm4 during microtubule-dependent mRNA transport in the plant pathogen Ustilago maydis

• Applicant: Professor Dr. Michael Feldbrügge
• Subject Area: Plant Genetics and Genomics ; Microbial Ecology and Applied Microbiology
• Term: from 2012 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 222261341

• Final Report Year: 2017

Final Report Abstract

During infection the fungus Ustilago maydis switches from yeast-like to hyphal growth. Hyphae grow unipolarly by expanding at the apical pole and inserting basal septa in regular time intervals. During this phase long-distance transport along the microtubule cytoskeleton is essential. Important cargos are mRNAs, associated ribosomes and septins. A key factor involved is Rrm4 transporting mRNAs on the cytoplasmic surface of early endosomes. Studying potential target mRNAs of Rrm4 revealed that the regulator Did2 of the ESCRT complex (endosomal sorting complex required for transport) is needed for unipolar growth of hyphae. The protein localises to Rrm4-positive endosomes but unexpectedly the endosomal localisation was not strictly dependent on the Rrm4-dependent transport machinery. Detailed analysis revealed a reciprocal relationship. Did2 is needed for the correct processive movement of Rrm4. Altered endosomal Rrm4 movement is most likely due to altered recruitment of the adaptor protein Upa1. Consistently, besides Rrm4 also the endosomal trafficking of Pab1 and septin Cdc3 was affected. The defects were not restricted to Rrm4-dependent endosomal transport. The use of Rab marker proteins and specific biosensors for early endosomes revealed that in did2 strains amount and processive movement of endosomes was disturbed. Defects in processive movement were due to altered recruitment of Kinesin-3 type motor Kin3 for plus-end directed transport as well as cytoplasmic dynein for minus-end directed transport. This was most likely due to altered membrane composition of early endosomes. Importantly, loss of Did2 caused a substantially increased shuttling of late endosomal marker Rab7 as well as cargo protein Prc1. This suggests that indeed membrane composition and endosomal maturation was disturbed. In essence, Did2 is needed for coordination of endocytic trafficking and endosomal long distance transport by regulating the correct membrane composition of maturing early endosomes. Interestingly, similar results seem to apply for neuronal processes since Rab5 and ESCRT-II have been implicated in RNA transport in neuronal cells.

Publications

• (2017) The ESCRT regulator Did2 maintains the balance between long-distance endosomal transport and endocytic trafficking. PLoS genetics 13 (4) e1006734
  Haag, Carl; Pohlmann, Thomas; Feldbrügge, Michael
  
• (2014) mRNA transport meets membrane traffic. Trends in Genet. 30: 408-417
  R.-P. Jansen, D. Niessing, S. Baumann and M. Feldbrügge
  
• (2015) A FYVE zinc finger domain protein specifically links mRNA transport to endosome trafficking. eLife;10.7554/eLife.06041
  T. Pohlmann, S. Baumann, C. Haag, M. Albrecht and M. Feldbrügge
  
• (2015) Membrane-coupled mRNA trafficking in fungi. Annu. Rev. Microbiol. 69: 265-281
  C. Haag, B. Steuten, and M. Feldbrügge