Meiotic Clade AAA ATPases: Protein Polymer Disassembly Machines

Abstract

Meiotic clade AAA ATPases, which were initially grouped on the basis of phylogenetic classification of their AAA ATPase cassette, include four relatively well characterized family members, Vps4, spastin, katanin, and fidgetin. These enzymes all function to disassemble specific polymeric protein structures, with Vps4 disassembling the ESCRT-III polymers that are central to the many membrane-remodeling activities of the ESCRT pathway, and spastin, katanin p60 and fidgetin affecting multiple aspects of cellular dynamics by severing microtubules. They share a common domain architecture that features an N-terminal MIT domain followed by a single AAA ATPase cassette. Meiotic clade AAA ATPases function as hexamers that can cycle between the active assembly and inactive monomers/dimers in a regulated process, and appear to disassemble their polymeric substrates by translocating subunits through the central pore of their hexameric ring. Recent studies with Vps4 have shown that nucleotide-induced asymmetry is a requirement for substrate binding to the pore loops, and that recruitment to the protein lattice via MIT domains also relieves auto-inhibition and primes the AAA ATPase cassettes for substrate binding. The most striking, unifying feature of meiotic clade AAA ATPases may be their MIT domain, which is a module that is found in a wide variety of proteins that localize to ESCRT-III polymers. Spastin also displays an adjacent microtubule-binding sequence, and the presence of both ESCRT-III and microtubule binding elements may underlie the recent findings that the ESCRT-III disassembly function of Vps4 and the microtubule-severing function of spastin, and potentially katanin and fidgetin, are highly coordinated.

Structures, Functions, and Dynamics of ESCRT-III/Vps4 Membrane Remodeling and Fission Complexes

“…This generalized model is supported by conservation of pore loop residues and other key structural elements (Monroe & Hill 2016, Sauer & Baker 2011) and by a series of recent structures of other AAA+ ATPases in complex with mixed polypeptide substrates (Deville et al 2017, Gates et al 2017, Puchades et al 2017, Ripstein et al 2017). These structures collectively represent a long-awaited breakthrough in our understanding of this large and important class of cellular machines (Harrison 2004), whose well-known representatives include essential activities like the 19S proteasome, NSF, and p97/Cdc48.…”

“…Vps4 enzymes drive the dynamic exchange of subunits into and out of ESCRT-III filaments and ultimately recycle the subunits back into the cytoplasm, thereby harnessing the energy of ATP hydrolysis to power ESCRT-dependent membrane fission reactions (Monroe & Hill 2016). Vps4 enzymes contain three different structural elements: ( a ) an N-terminal MIT domain that binds the tails of ESCRT-III proteins (Figures 2 c and 5 a ); ( b ) a central ATPase cassette comprising large and small domains that mediate hexamerization and ATP hydrolysis; and ( c ) a β-domain insert within the small ATPase domain that binds LIP5 (Vta1), an ATPase activator and ESCRT-III-binding protein.…”

“…These structures revealed that Vps4 forms an asymmetric ring hexamer that binds the exposed tails of ESCRT-III subunits and translocates the subunits through the constricted central pore of the hexamer. Importantly, this mechanism may be conserved across AAA ATPases of the meiotic clade, including SPASTIN, KATANIN, and FIDGETIN, which are best characterized as microtubule-severing enzymes that bind the tails of tubulin subunits and extract them from the polymer (Monroe & Hill 2016). …”

Reverse-topology membrane scission by the ESCRT proteins

“…There is a consensus that VPS4 71, 72 has an essential role in recycling ESCRT-III subunits, but is probably does more than that. The questions surrounding the precise role of VPS4 are at the crux of much of the uncertainty about how ESCRTs actually sever membranes.…”

Designed proteins induce the formation of nanocage-containing extracellular vesicles

“…Examples included the Equine Infectious Anemia Virus (EIAV) p9 Gag element, which recruits ALIX 19 , and an N-terminal element from Ebola Virus VP40 protein, which recruits both TSG101/ESCRT-I and NEDD4 protein family members 20 . Release was ESCRT-dependent in all cases because EPN production was strongly inhibited by overexpression of a dominant inhibitory version of the VPS4 ATPase that powers the ESCRT pathway 21 .…”