Swanson Stephanie, Ioerger Thomas R, Rigel Nathan W, Miller Brittany K, Braunstein Miriam, Sacchettini James C
Department of Biochemistry & Biophysics, Texas A&M University, College Station, Texas, USA.
Department of Computer Science and Engineering, Texas A&M University, College Station, Texas, USA.
J Bacteriol. 2015 Dec 14;198(4):720-30. doi: 10.1128/JB.00696-15.
While SecA is the ATPase component of the major bacterial secretory (Sec) system, mycobacteria and some Gram-positive pathogens have a second paralog, SecA2. In bacteria with two SecA paralogs, each SecA is functionally distinct, and they cannot compensate for one another. Compared to SecA1, SecA2 exports a distinct and smaller set of substrates, some of which have roles in virulence. In the mycobacterial system, some SecA2-dependent substrates lack a signal peptide, while others contain a signal peptide but possess features in the mature protein that necessitate a role for SecA2 in their export. It is unclear how SecA2 functions in protein export, and one open question is whether SecA2 works with the canonical SecYEG channel to export proteins. In this study, we report the structure of Mycobacterium tuberculosis SecA2 (MtbSecA2), which is the first structure of any SecA2 protein. A high level of structural similarity is observed between SecA2 and SecA1. The major structural difference is the absence of the helical wing domain, which is likely to play a role in how MtbSecA2 recognizes its unique substrates. Importantly, structural features critical to the interaction between SecA1 and SecYEG are preserved in SecA2. Furthermore, suppressor mutations of a dominant-negative secA2 mutant map to the surface of SecA2 and help identify functional regions of SecA2 that may promote interactions with SecYEG or the translocating polypeptide substrate. These results support a model in which the mycobacterial SecA2 works with SecYEG.
SecA2 is a paralog of SecA1, which is the ATPase of the canonical bacterial Sec secretion system. SecA2 has a nonredundant function with SecA1, and SecA2 exports a distinct and smaller set of substrates than SecA1. This work reports the crystal structure of SecA2 of Mycobacterium tuberculosis (the first SecA2 structure reported for any organism). Many of the structural features of SecA1 are conserved in the SecA2 structure, including putative contacts with the SecYEG channel. Several structural differences are also identified that could relate to the unique function and selectivity of SecA2. Suppressor mutations of a secA2 mutant map to the surface of SecA2 and help identify functional regions of SecA2 that may promote interactions with SecYEG.
虽然SecA是主要细菌分泌（Sec）系统的ATP酶成分，但分枝杆菌和一些革兰氏阳性病原体有第二个旁系同源物SecA2。在具有两个SecA旁系同源物的细菌中，每个SecA功能不同，且它们不能相互补偿。与SecA1相比，SecA2输出一组不同且数量较少的底物，其中一些底物在毒力方面起作用。在分枝杆菌系统中，一些SecA2依赖性底物缺乏信号肽，而其他底物含有信号肽，但在成熟蛋白中具有一些特征，这使得SecA2在它们的输出过程中发挥作用。目前尚不清楚SecA2在蛋白质输出中如何发挥作用，一个悬而未决的问题是SecA2是否与经典的SecYEG通道协同作用来输出蛋白质。在本研究中，我们报道了结核分枝杆菌SecA2（MtbSecA2）的结构，这是任何SecA2蛋白的首个结构。在SecA2和SecA1之间观察到高度的结构相似性。主要的结构差异是缺少螺旋翼结构域，这可能在MtbSecA2识别其独特底物的方式中起作用。重要的是，SecA1与SecYEG之间相互作用的关键结构特征在SecA2中得以保留。此外，显性负性secA2突变体的抑制突变定位在SecA2的表面，并有助于确定SecA2的功能区域，这些区域可能促进与SecYEG或转运多肽底物的相互作用。这些结果支持了一种模型，即分枝杆菌SecA2与SecYEG协同作用。
SecA2是SecA1的旁系同源物，SecA1是经典细菌Sec分泌系统的ATP酶。SecA2与SecA1具有非冗余功能，并且SecA2输出的底物组与SecA1不同且数量较少。这项工作报道了结核分枝杆菌SecA2的晶体结构（这是针对任何生物体报道的首个SecA2结构）。SecA1的许多结构特征在SecA2结构中得以保留，包括与SecYEG通道的假定接触。还确定了几个可能与SecA2的独特功能和选择性有关的结构差异。secA2突变体的抑制突变定位在SecA2的表面，并有助于确定SecA2的功能区域，这些区域可能促进与SecYEG的相互作用。
