The PII family comprises a group of widely distributed signal transduction proteins ubiquitous in prokaryotes and in the chloroplasts of plants. PII proteins sense the levels of key metabolites ATP, ADP, and 2-oxoglutarate, which affect the PII protein structure and thereby the ability of PII to interact with a range of target proteins. Here, we performed multiple ligand fishing assays with the PII protein orthologue GlnZ from the plant growth-promoting nitrogen-fixing bacterium Azospirillum brasilense to identify 37 proteins that are likely to be part of the PII protein-protein interaction network. Among the PII targets identified were enzymes related to nitrogen and fatty acid metabolism, signaling, coenzyme synthesis, RNA catabolism, and transcription. Direct binary PII-target complex was confirmed for 15 protein complexes using pulldown assays with recombinant proteins. Untargeted metabolome analysis showed that PII is required for proper homeostasis of important metabolites. Two enzymes involved in c-di-GMP metabolism were among the identified PII targets. A PII-deficient strain showed reduced c-di-GMP levels and altered aerotaxis and flocculation behavior. These data support that PII acts as a major metabolic hub controlling important enzymes and the homeostasis of key metabolites such as c-di-GMP in response to the prevailing nutritional status.

中文翻译：

---

蛋白质-蛋白质相互作用网络揭示了信号转导蛋白PII在控制巴西偶氮螺旋藻c-di-GMP体内平衡中的新作用。

PII家族包含一组在原核生物和植物叶绿体中普遍存在的广泛分布的信号转导蛋白。PII蛋白可检测关键代谢物ATP，ADP和2-氧戊二酸的水平，这会影响PII蛋白的结构，从而影响PII与多种靶蛋白相互作用的能力。在这里，我们使用了促进植物生长的固氮细菌巴西细螺旋藻（Azospirillum brasilense）的PII蛋白直向同源GlnZ进行了多个配体捕捞试验鉴定可能属于PII蛋白质-蛋白质相互作用网络一部分的37种蛋白质。在鉴定出的PII目标中，有与氮和脂肪酸代谢，信号传导，辅酶合成，RNA分解代谢和转录有关的酶。使用带有重组蛋白的下拉测定法，确认了15种蛋白复合物的直接二元PII-靶标复合物。非靶向代谢组分析表明，重要代谢物的体内稳态需要PII。参与c-di-GMP代谢的两种酶属于确定的PII目标。缺乏PII的菌株显示出降低的c-di-GMP水平并改变了气浮性和絮凝行为。