雙元件系統是廣泛出現於原核生物中的訊息傳遞機制，可幫助細胞感受周遭環境改變的刺激並對此進行調控反應。在綠膿桿菌此一重要的機會致病菌中，由磷酸轉移酶HptB所參與的雜合型雙元件系統被發現與調控菌體的泳動能力和生物膜形成有關有關。為了瞭解磷酸接收區與磷酸轉移酶之間訊息傳遞的機制，我們把同步輻射X光繞射分別得到兩蛋白晶體的高解析度繞射數據，並藉此解構出磷酸轉移酶HptB與訊息感知酶PA1611之磷酸接受區的晶體結構。分析後發現這兩種蛋白質的晶體結構均呈現典型的摺疊構型。HptB由α螺旋2到α螺旋5摺疊成細長的四螺旋束，其N端由α螺旋1覆蓋。高度保留性的活性氨基酸His57位於α3接近中間的位置，其側鍊曝露於溶液環境中。PA1611REC呈現(β/α)5構型，中心部份的五個β摺板被周圍的五個α螺旋環繞，帶負電的活性區中有一鎂離子，其與三個氨基酸(Asp522、Asp565、Arg567)間有交互作用力。藉由電腦模擬的方式，建構了此磷酸接收區與磷酸轉移酶在進行訊息傳遞時可能形成的複合體結構並了解其間之交互作用，並發現與其他複合體相比，參與交互作用的氨基酸數量較少，作用力也有所不同。這有助於進一步瞭解綠膿桿菌中的雜合型雙元件系統，並有潛力引領針對綠膿桿菌感染的新療法之開發。

In Pseudomonas aeruginosa, an important opportunistic pathogen causing numerous acute and chronic infections, the hybrid two-component system (TCS) regulates the swarming ability and biofilm formation with a multistep phosphorelay and consists of hybrid sensor histidine kinase (HK), histidine-containing phosphotransfer protein (Hpt) and response regulator (RR). In this study, the crystal structures of HptB (PA3345) and the receiver domain of HK PA1611 (PA1611REC) were determined, respectively, of P. aeruginosa to elucidate their interactions for the transfer of the phosphoryl group. The structure of HptB folds into an elongated four-helix bundle composed of helices α2 to α5 and covered by a short N-terminal helix α1. The imidazole side chain of the conserved active-site histidine residue, His57, is located near the middle of helix α3 and protrudes to a solvent. The structure of PA1611REC possesses a conventional (β/α)5 topology with 5 α-helices surrounding 5 parallel β-sheets. The divalent Mg2+ ion is located in the negatively charged active-site cleft and interacts with Asp522, Asp565 and Arg567.The HptB-PA1611REC complex is further modeled to analyze the binding surface and interactions between the two proteins. The model shows a shape complementarity between the convex surface of PA1611REC and the kidney-shaped HptB with fewer residues and a different network involved in interactions compared to other TCS complexes, such as SLN1-R1/YPD1 from Saccharomyces cerevisiae and AHK5RD/AHP1 from Arabidopsis thaliana. These structural results provide a better understanding of TCS in P. aeruginosa and potentially leads to a discovery of the new treatment for infection.

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