(A) In the presence of ATP, IMPDH2 assembles extended octamer filaments. Binding of GTP leads to assembly of compressed octamer filaments that remain partially active where the tetramer is in a flat conformation. In the presence of very high GTP concentrations, the tetramer is completely inhibited and enters a bowed conformation which promotes disassembly of the filament into free octamers [5] (B) For canonical IMPDH1, binding of ATP drives assembly of a filament composed of extended octamers. In the presence of GTP, canonical IMPDH1 assembles into a filament with the small interface made of fully compressed octamers that are mostly inhibited and have the bowed tetramer conformation. Figure adapted from [6].