Assembly-mediated activation of the SIR2-HerA supramolecular complex for anti-phage defense.

SIR2-HerA, a bacterial two-protein anti-phage defense system, induces bacterial death by depleting NAD+ upon phage infection. Biochemical reconstitution of SIR2, HerA, and the SIR2-HerA complex reveals a dynamic assembly process. Unlike other ATPases, HerA can form various oligomers, ranging from dimers to nonamers. When assembled with SIR2, HerA forms a hexamer and converts SIR2 from a nuclease to an NAD+ hydrolase, representing an unexpected regulatory mechanism mediated by protein assembly. Furthermore, high concentrations of ATP can inhibit NAD+ hydrolysis by the SIR2-HerA complex. Cryo-EM structures of the SIR2-HerA complex reveal a giant supramolecular assembly up to 1 MDa, with SIR2 as a dodecamer and HerA as a hexamer, crucial for anti-phage defense. Unexpectedly, the HerA hexamer resembles a spiral staircase and exhibits helicase activities toward dual-forked DNA. Together, we reveal the supramolecular assembly of SIR2-HerA as a unique mechanism for switching enzymatic activities and bolstering anti-phage defense strategies.

Molecular basis of Gabija anti-phage supramolecular assemblies.

As one of the most prevalent anti-phage defense systems in prokaryotes, Gabija consists of a Gabija protein A (GajA) and a Gabija protein B (GajB). The assembly and function of the Gabija system remain unclear. Here we present cryo-EM structures of Bacillus cereus GajA and GajAB complex, revealing tetrameric and octameric assemblies, respectively. In the center of the complex, GajA assembles into a tetramer, which recruits two sets of GajB dimer at opposite sides of the complex, resulting in a 4:4 GajAB supramolecular complex for anti-phage defense. Further biochemical analysis showed that GajA alone is sufficient to cut double-stranded DNA and plasmid DNA, which can be inhibited by ATP. Unexpectedly, the GajAB displays enhanced activity for plasmid DNA, suggesting a role of substrate selection by GajB. Together, our study defines a framework for understanding anti-phage immune defense by the GajAB complex.

Cryo-EM structure of human hexameric MCM2-7 complex.

The central step in the initiation of eukaryotic DNA replication is the loading of the minichromosome maintenance 2-7 (MCM2-7) complex, the core of the replicative DNA helicase, onto chromatin at replication origin. Here, we reported the cryo-EM structure of endogenous human single hexameric MCM2-7 complex with a resolution at 4.4 Å, typically an open-ring hexamer with a gap between Mcm2 and Mcm5. Strikingly, further analysis revealed that human MCM2-7 can self-associate to form a loose double hexamer which potentially implies a novel mechanism underlying the MCM2-7 loading in eukaryote. The high-resolution cryo-EM structure of human MCM2-7 is critical for understanding the molecular mechanisms governing human DNA replication, especially the MCM2-7 chromatin loading and pre-replicative complex assembly.