PtuA and PtuB assemble into an inflammasome-like oligomer for anti-phage defense.

Escherichia coli Septu system, an anti-phage defense system, comprises two components: PtuA and PtuB. PtuA contains an ATPase domain, while PtuB is predicted to function as a nuclease. Here we show that PtuA and PtuB form a stable complex with a 6:2 stoichiometry. Cryo-electron microscopy structure of PtuAB reveals a distinctive horseshoe-like configuration. PtuA adopts a hexameric arrangement, organized as an asymmetric trimer of dimers, contrasting the ring-like structure by other ATPases. Notably, the three pairs of PtuA dimers assume distinct conformations and fulfill unique roles in recruiting PtuB. Our functional assays have further illuminated the importance of the oligomeric assembly of PtuAB in anti-phage defense. Moreover, we have uncovered that ATP molecules can directly bind to PtuA and inhibit the activities of PtuAB. Together, the assembly and function of the Septu system shed light on understanding other ATPase-containing systems in bacterial immunity.

Molecular mechanisms of Holliday junction branch migration catalyzed by an asymmetric RuvB hexamer.

The Holliday junction (HJ) is a DNA intermediate of homologous recombination, involved in many fundamental physiological processes. RuvB, an ATPase motor protein, drives branch migration of the Holliday junction with a mechanism that had yet to be elucidated. Here we report two cryo-EM structures of RuvB, providing a comprehensive understanding of HJ branch migration. RuvB assembles into a spiral staircase, ring-like hexamer, encircling dsDNA. Four protomers of RuvB contact the DNA backbone with a translocation step size of 2 nucleotides. The variation of nucleotide-binding states in RuvB supports a sequential model for ATP hydrolysis and nucleotide recycling, which occur at separate, singular positions. RuvB's asymmetric assembly also explains the 6:4 stoichiometry between the RuvB/RuvA complex, which coordinates HJ migration in bacteria. Taken together, we provide a mechanistic understanding of HJ branch migration facilitated by RuvB, which may be universally shared by prokaryotic and eukaryotic organisms.

It takes two to Tango: Two gates orchestrate the opening of human TRPM2.

TRPM2 is a calcium permeable non-selective cation channel involved in many important physiological processes and has divergent gating mechanisms across species. Structural studies have revealed that TRPM2 is gated by adenosine 5'-diphosphoribose that binds to the cytosolic domains of TRPM2 and calcium ions that are coordinated by residues in the transmembrane domain. However, the selectivity filter of human TRPM2 remains elusive due to the poor resolution in this region. In a recent manuscript published in Cell Reports, Yu et al. present unexpected dual roles of the selectivity filter in human TRPM2 by determining a high-resolution structure of human TRPM2 in lipid nanodiscs. This study provides unprecedented insights into the gating mechanism of human TRPM2.