DSR2, a Sir2 domain-containing protein, protects bacteria from phage infection by hydrolyzing NAD⁺. The enzymatic activity of DSR2 is triggered by the SPR phage tail tube protein (TTP), while suppressed by the SPbeta phage-encoded DSAD1 protein, enabling phages to evade the host defense. However, the molecular mechanisms of activation and inhibition of DSR2 remain elusive. Here, we report the cryo-EM structures of apo DSR2, DSR2-TTP-NAD⁺ and DSR2-DSAD1 complexes. DSR2 assembles into a head-to-head tetramer mediated by its Sir2 domain. The C-terminal helical regions of DSR2 constitute four partner-binding cavities with opened and closed conformation. Two TTP molecules bind to two of the four C-terminal cavities, inducing conformational change of Sir2 domain to activate DSR2. Furthermore, DSAD1 competes with the activator for binding to the C-terminal cavity of DSR2, effectively suppressing its enzymatic activity. Our results provide the mechanistic insights into the DSR2-mediated anti-phage defense system and DSAD1-dependent phage immune evasion.

DSR2 是一种含有 Sir2 结构域的蛋白质，通过水解 NAD ⁺来保护细菌免受噬菌体感染。 DSR2 的酶活性由 SPR 噬菌体尾管蛋白 (TTP) 触发，同时被 SPbeta 噬菌体编码的 DSAD1 蛋白抑制，使噬菌体能够逃避宿主防御。然而，DSR2 激活和抑制的分子机制仍不清楚。在这里，我们报告了 apo DSR2、DSR2-TTP-NAD ⁺和 DSR2-DSAD1 复合物的冷冻电镜结构。 DSR2 组装成由 Sir2 结构域介导的头对头四聚体。 DSR2 的 C 端螺旋区域构成四个具有开放和闭合构象的配偶体结合腔。两个 TTP 分子与四个 C 末端空腔中的两个结合，诱导 Sir2 结构域的构象变化以激活 DSR2。此外，DSAD1 与激活剂竞争结合 DSR2 的 C 端空腔，有效抑制其酶活性。我们的结果为 DSR2 介导的抗噬菌体防御系统和 DSAD1 依赖性噬菌体免疫逃避提供了机制见解。