Human CTP synthase filament structure reveals the active enzyme conformation

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• The enzyme CTP synthase (CTPS) is essential and conserved across species, forming filaments in bacteria and eukaryotes.
• In bacteria, CTPS filament formation inhibits enzyme activity, playing a role in nucleotide homeostasis.
• The functional consequences of CTPS polymerization in humans were previously unknown.

Purpose of the Study:

• To investigate the structural basis and functional consequences of human CTP synthase (CTPS) filament formation.
• To compare the structure and function of bacterial and human CTPS filaments.
• To elucidate the allosteric mechanisms governing CTPS assembly and regulation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structures of bacterial and human CTPS filaments.
• Structural analysis to compare the architecture and protomer conformations of different CTPS filaments.
• Functional assays to assess the catalytic activity of human CTPS in filament and non-filament states.

Main Results:

• Human CTPS filaments exhibit a distinct architecture and protomer conformation compared to bacterial filaments.
• Polymerization of human CTPS leads to an increase in its catalytic activity, a novel finding.
• The cryo-EM structure of the human CTPS filament reveals a previously uncharacterized active conformation.
• Allosteric regulation mechanisms involving conserved conformational equilibrium were elucidated.

Conclusions:

• Human CTPS filament formation enhances enzyme activity, contrasting with bacterial CTPS where it is inhibitory.
• The distinct structures of bacterial and human CTPS filaments explain the divergent effects of polymerization on enzyme function.
• These findings challenge the notion that metabolic filaments primarily serve as inactive enzyme storage.
• Allosteric regulation of CTPS polymerization by ligands may be a fundamental mechanism for other metabolic filaments.