Vermaas Josh V, Tajkhorshid Emad
Center for Biophysics and Quantitative Biology, Department of Biochemistry, and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Biochemistry. 2017 Jan 10;56(1):281-293. doi: 10.1021/acs.biochem.6b00468. Epub 2016 Dec 20.
Synaptotagmin (Syt) is a membrane-associated protein involved in vesicle fusion through the SNARE complex that is found throughout the human body in 17 different isoforms. These isoforms have two membrane-binding C2 domains, which sense Ca and thereby promote anionic membrane binding and lead to vesicle fusion. Through molecular dynamics simulations using the highly mobile membrane mimetic acclerated bilayer model, we have investigated how small protein sequence changes in the Ca-binding loops of the C2 domains may give rise to the experimentally determined difference in binding kinetics between Syt-1 and Syt-7 isoforms. Syt-7 C2 domains are found to form more close contacts with anionic phospholipid headgroups, particularly in loop 1, where an additional positive charge in Syt-7 draws the loop closer to the membrane and causes the anchoring residue F167 to insert deeper into the bilayer than the corresponding methionine in Syt-1 (M173). By performing additional replica exchange umbrella sampling calculations, we demonstrate that these additional contacts increase the energetic cost of unbinding the Syt-7 C2 domains from the bilayer, causing them to unbind more slowly than their counterparts in Syt-1.
突触结合蛋白（Syt）是一种与膜相关的蛋白质，通过SNARE复合体参与囊泡融合，在人体中以17种不同的异构体形式存在。这些异构体具有两个膜结合C2结构域，可感知钙离子，从而促进阴离子膜结合并导致囊泡融合。通过使用高流动性膜模拟加速双层模型进行分子动力学模拟，我们研究了C2结构域钙离子结合环中微小的蛋白质序列变化如何导致实验测定的Syt-1和Syt-7异构体之间结合动力学的差异。研究发现，Syt-7的C2结构域与阴离子磷脂头部形成更多紧密接触，特别是在环1中，Syt-7中额外的正电荷使该环更靠近膜，并导致锚定残基F167比Syt-1中的相应甲硫氨酸（M173）更深地插入双层膜中。通过进行额外的副本交换伞形采样计算，我们证明这些额外的接触增加了Syt-7的C2结构域从双层膜上解离的能量成本，导致它们比Syt-1中的对应结构域解离得更慢。
