The universally conserved enzyme CTP synthase (CTPS) forms filaments in bacteria and eukaryotes. In bacteria polymerization inhibits CTPS activity and is required for nucleotide homeostasis. Here we show that human CTPS polymerization increases catalytic activity. The cryoEM structures of bacterial and human CTPS filaments differ dramatically in overall architecture and in the conformation of the CTPS protomer, explaining the divergent consequences of polymerization on activity. The filament structure of human CTPS is the first full-length structure of the human enzyme and reveals a novel active conformation. The filament structures elucidate allosteric mechanisms of assembly and regulation that rely on a conserved conformational equilibrium. This may provide a mechanism for increasing human CTPS activity in response to metabolic state, and challenges the assumption that metabolic filaments are generally storage forms of inactivated enzymes. Allosteric regulation of CTPS polymerization by ligands likely represents a fundamental mechanism underlying assembly of other metabolic filaments.

CTP synthase (CTPS) is one of several metabolic enzymes recently found to form micron-scale filaments in prokaryotes and eukaryotes. CTPS is an essential, universally conserved enzyme responsible for catalyzing the rate-limiting step in CTP biosynthesis -conversion of UTP to CTP in an ATP-dependent process. CTP is necessary for cellular function, being a component of DNA and RNA, and also critical in phospholipid synthesis. Previously, we showed