Dynamic functional assembly of the Torsin AAA+ ATPase and its modulation by LAP1

TorsinA is an essential AAA+ ATPase requiring LAP1 or LULL1 as cofactors. The dynamics of the Torsin/cofactor system remain poorly understood, with previous models invoking Torsin/cofactor assemblies with fixed stoichiometries. Here we demonstrate that TorsinA assembles into homotypic oligomers in the presence of ATP. Torsin variants mutated at the "back" interface disrupt homo-oligomerization but still show robust ATPase activity in the presence of its cofactors. These Torsin mutants are severely compromised in their ability to rescue nuclear envelope defects in Torsin-deficient cells, suggesting that TorsinA homo-oligomers play a key role in vivo. Engagement of the oligomer by LAP1 triggers ATP hydrolysis and rapid complex disassembly. Thus the Torsin complex is a highly dynamic assembly whose oligomeric state is tightly controlled by distinctively localized cellular cofactors. Our discovery that LAP1 serves as a modulator of the oligomeric state of an AAA+ protein establishes a novel means of regulating this important class of oligomeric ATPases.