Department of Physics and Center for Molecular Study of Condensed Soft Matter, Illinois Institute of Technology, 3440 S Dearborn St, Chicago, IL, 60616, USA.
Sci Rep. 2017 Oct 13;7(1):13119. doi: 10.1038/s41598-017-13135-3.
Pili are elongated structures that protrude from bacteria and increase their virulence. The Streptococcus pnuenomae pilus island 1 pili are composed of three subunits, RrgA, RrgB, and RrgC, and are assembled by three class C sortase C (SrtC) enzymes: SrtC-1, SrtC-2, and SrtC-3. Pilin subunits are recognized by SrtC proteins through a pentapeptide sorting signal, and while previous studies have sought to characterize the selectivities of SrtC isoforms for these subunits, the molecular mechanisms underlying these interactions remain unclear. Here, we report a series of molecular dynamics simulations of each SrtC enzyme with the sorting signals of RrgA, RrgB, and RrgC to determine the structural and thermodynamic basis of pilin recognition. Results show that, in accordance with previous studies, both SrtC-1 and SrtC-3 are selective for RrgB, while SrtC-2 is selective for RrgA. This specificity is tuned by the sorting signal binding conformation in which the first two residue sidechains complement hydrophobic residues around the active site, while the third residue projects away from the catalytic triad and makes specific interactions based on its charge and reach. Together, these results provided atomic-scale descriptions of the SrtC substrate selectivity mechanisms and extend the emerging model of pilin construction in S. pnuenomae.
菌毛是从细菌中伸出的细长结构，可增强其毒力。肺炎链球菌 pilus 岛 1 菌毛由三个亚基 RrgA、RrgB 和 RrgC 组成，由三个 C 类 sortase C（SrtC）酶组装：SrtC-1、SrtC-2 和 SrtC-3。SrtC 蛋白通过五肽分拣信号识别菌毛亚基，尽管先前的研究试图表征 SrtC 同工酶对这些亚基的选择性，但这些相互作用的分子机制仍不清楚。在这里，我们报告了一系列与 RrgA、RrgB 和 RrgC 的分选信号的每个 SrtC 酶的分子动力学模拟，以确定菌毛识别的结构和热力学基础。结果表明，与先前的研究一致，SrtC-1 和 SrtC-3 都对 RrgB 具有选择性，而 SrtC-2 对 RrgA 具有选择性。这种特异性是通过分选信号结合构象来调节的，其中前两个残基侧链与活性位点周围的疏水性残基互补，而第三个残基远离催化三联体，并根据其电荷和范围进行特异性相互作用。这些结果提供了 SrtC 底物选择性机制的原子尺度描述，并扩展了肺炎链球菌中菌毛构建的新兴模型。
