嗜热链球菌dlt操纵子终端亚基stDltD的晶体结构

细胞表面多聚物的酰基跨膜修饰对增强细菌的致病性至关重要. DltA/B/C/D操纵子介导的脂磷壁酸（LTA）D-丙酰化修饰是革兰氏阳性菌中重要的一类后修饰，其调节膜内外的电荷平衡. DltA/B/C/D操纵子主要由DltA、DltB、DltC 和DltD四种蛋白质亚基组成，其催化机制与结构在生物进化中高度保守. DltA/DltC介导的胞内D-丙酰胺的转移机理已有深入的研究，而跨膜O-酰基转移酶DltB和dlt操纵子末端DltD介导的跨膜催化过程并不清楚. 本文解析了来源于嗜热链球菌 （S. thermophilus）中stDltD膜外结构域2.94 ?分辨率的晶体三维结构. 结构比对分析表明，stDltD是dlt操纵子终端的酰基转移酶，属于SGNH-like家族，stDltD的活性中心，包括4个blocks和催化三联体，都保守存在于多种革兰氏阳性病原菌中. 此外，结构叠合分析表明，stDltD催化中心形成的正电荷窝沟正好可以结合一个脂磷壁酸骨架的单体即甘油磷酸分子. 在前人的研究基础上，我们提出了一个由Dlt/A/B/C/D操纵子介导跨膜D-丙酰化修饰的工作模型. 本研究对进一步阐明DltD的生物学功能以及Dlt/A/B/C/D操纵子酰基跨膜修饰的分子机制具有重要意义.

Crystal Structure of an O-acyltransfer Terminal Protein stDltD and Its Implications for dlt Operon-mediated D-alanylation of S. thermophilus

In bacteria, the acyl transmembrane modification of cell-surface polymers is a common feature to strengthen their pathogenic potential. The dlt operon-mediated D-alanylation of lipoteichoic acids (LTAs) is an important post-modification to adjust charge balance in Gram-positive bacteria. Four proteins of DltA/B/C/D were identified to be essential for LTA D-alanine incorporation. Though the process of D-alanine transfer by cytoplasmic DltA/DltC has been largely probed, transmembrane catalysis by MBOAT protein DltB and the terminal player DltD is yet to be defined. Here, the crystal structure of stDltD was determined from S. thermophilus at 2.94 ? resolution. On the basis of the structure comparison, DltD was considered as the terminal acyltransferase of the dlt operon, and it belonged to the SGNH-like family. An stDltD active center, including four blocks and a catalytic triad, conservatively exists in various Gram-positive pathogens. In addition, structural analysis showed that the stDltD catalytic center formed a strong positively charged groove docking with a glycerolphosphate molecule. Combined with previous reports, an updated working model was proposed for cross-membrane D-alanylation mediated by the dlt operon. The structural evidence provides more implications to clarify the biological function of stDltD and the process of transmembrane acyl modification.