External The multi-tool AAA+ ATPase torsinA is a Rosetta Stone for understanding the molecular mechanisms of nuclear- cytoplasmic communication. Seminar details September 9, 2019, 12:00 pm @ Murray Seminar RoomDr G W Gant Luxton, University of Minnesota Host: Tomoyuki U Tanaka Abstract: Genetic mutations in the nuclear envelope-localized AAA+ ATPase torsinA cause a spectrum of poorly understood neurological diseases including DYT1 dystonia and severe arthrogryposis. While torsinA function is required for nuclear-cytoskeletal coupling and nuclear pore complex (NPC) biogenesis, how torsinA mediates these functions and their relationship to disease pathogenesis remain unclear. Here, we determined the functional assembly state of torsinA in the nuclear envelope of living cells using fluorescence fluctuation spectroscopy. In contrast to most AAA+ ATPases, which function as homo-hexamers, the in vivo homo-oligomerization of torsinA was limited to a homo-trimer. Unexpectedly, the deletion of its membrane-associating N-terminal domain (NTD) resulted in the assembly of torsinA homo-oligomers larger than expected for a homo-hexamer. By measuring the assembly states of constructs containing torsinA monomers covalently linked into artificial dimers, trimers, tetramers, pentamers, and hexamers, we revealed that torsinA polymerizes following the rate limiting step of dimerization at discreet sites within the nuclear envelope. Next, we investigated the physiological relevance of torsinA polymerization by testing its role during nuclear-cytoskeletal coupling and NPC biogenesis. Interestingly, we found that torsinA polymerization was required for nuclear-cytoskeletal coupling but dispensable for NPC biogenesis. Moreover, we determined that membrane-association was critical for NPC biogenesis but detrimental to nuclear-cytoskeletal coupling. In fact, nuclear-cytoskeletal coupling was prevented when the NTD was converted into a transmembrane domain or when a previously described proteolytic cleavage event that selectively removes the NTD was inhibited. Finally, we showed that the ability of torsinA to hydrolyze ATP was not important for NPC biogenesis, whereas it was essential for nuclear-cytoskeletal coupling. These results suggest that torsinA is a multi-tool AAA+ ATPase that acts via different assembly states to execute distinct functions within the nuclear envelope. In summary, our work identifies torsinA as a powerful Rosetta Stone for understanding the molecular mechanisms of nuclear-cytoplasmic communication.
