DOI

RelA/SpoT homologue family enzymes participate in controlling the cellular levels of the alarmone (p)ppGpp, thereby activating the stringent response and promoting survival under stress conditions. These proteins contain an N-terminal catalytic domain and a C-terminal regulatory domain. They catalyze both the synthesis of ppGpp/pppGpp from ATP and GDP/GTP and their hydrolysis to GDP/GTP and pyrophosphate. Here, we report the crystal structure of the N-terminal domain of Rel from Streptococcus equisimilis in complex with pppGpp at 3.2 Å resolution. The asymmetric unit contains a dimer with asymmetric ligation: pppGpp occupies only the synthetase site in one monomer, whereas in the other monomer, it is bound in both the hydrolase and synthetase sites. The two monomers exhibit distinct conformational states, with pronounced rearrangements of the flexible loops surrounding the binding pockets, including the α2/α3 and α8/α9 loops that act as steric gates. Molecular dynamics simulations support the dual binding arrangement and reveal additional probable transient binding sites, including a region in the linker between hydrolase and synthetase subdomains. These findings provide a structural framework for understanding how pppGpp binding modulates the opposing catalytic activities of bifunctional Rel enzymes and suggest possible mechanisms for (p)ppGpp-mediated autoregulation.
Original languageEnglish
Article number5509
JournalInternational Journal of Molecular Sciences
Volume27
Issue number12
DOIs
StatePublished - 18 Jun 2026

    Research areas

  • Bacterial Proteins/chemistry, Binding Sites, Catalytic Domain, Crystallography, X-Ray, Guanosine Pentaphosphate/metabolism, Ligases/chemistry, Molecular Dynamics Simulation, Protein Binding, Protein Conformation, Protein Domains, Pyrophosphatases/chemistry, Streptococcus/enzymology, RelA/SpoT homolog, alarmone, stringent response, RelSeq, pppGpp, bifunctional enzyme, molecular dynamics, nanoDSF, crystal structure, X-ray

ID: 156129158