Coupled structural transitions enable highly cooperative regulation of human CTPS2 filaments

Many enzymes assemble into defined oligomers, providing a mechanism for cooperatively regulating enzyme activity. Recent studies in tissues, cells, and in vitro have described a mode of regulation in which enzyme activity is modulated by polymerization into large-scale filaments1-5. Enzyme polymerization is often driven by binding to substrates, products, or allosteric regulators, and tunes enzyme activity by locking the enzyme in high or low activity states1-5. Here, we describe a unique, ultrasensitive form of polymerization-based regulation employed by human CTP synthase 2 (CTPS2). High-resolution cryoEM structures of active and inhibited CTPS2 filaments reveal the molecular basis of this regulation. Rather than selectively stabilizing a single conformational state, CTPS2 filaments dynamically switch between active and inactive filament forms in response to changes in substrate and product levels. Linking the conformational state of many CTPS2 subunits in a filament results in highly cooperative regulation, greatly exceeding the limits of cooperativity for the CTPS2 tetramer alone. The structures also reveal a link between conformational state and control of ammonia channeling between the enzymes two active sites. This filament-based mechanism of enhanced cooperativity demonstrates how the widespread phenomenon of enzyme polymerization can be adapted to achieve different regulatory outcomes.