Structural Studies Division, Medical Research Council Laboratory of Molecular Biology, Cambridge Biomedical Campus, CB2 0QH Cambridge, United Kingdom.
Structural and Computational Biology Unit, European Molecular Biology Laboratory, 69117 Heidelberg, Germany.
Proc Natl Acad Sci U S A. 2021 Jan 19;118(3). doi: 10.1073/pnas.2020054118.
The CA (capsid) domain of immature HIV-1 Gag and the adjacent spacer peptide 1 (SP1) play a key role in viral assembly by forming a lattice of CA hexamers, which adapts to viral envelope curvature by incorporating small lattice defects and a large gap at the site of budding. This lattice is stabilized by intrahexameric and interhexameric CA-CA interactions, which are important in regulating viral assembly and maturation. We applied subtomogram averaging and classification to determine the oligomerization state of CA at lattice edges and found that CA forms partial hexamers. These structures reveal the network of interactions formed by CA-SP1 at the lattice edge. We also performed atomistic molecular dynamics simulations of CA-CA interactions stabilizing the immature lattice and partial CA-SP1 helical bundles. Free energy calculations reveal increased propensity for helix-to-coil transitions in partial hexamers compared to complete six-helix bundles. Taken together, these results suggest that the CA dimer is the basic unit of lattice assembly, partial hexamers exist at lattice edges, these are in a helix-coil dynamic equilibrium, and partial helical bundles are more likely to unfold, representing potential sites for HIV-1 maturation initiation.
不成熟 HIV-1 Gag 的 CA（衣壳）结构域和相邻的间隔肽 1（SP1）通过形成 CA 六聚体晶格在病毒组装中发挥关键作用，通过在出芽部位包含小晶格缺陷和大间隙来适应病毒包膜曲率。这种晶格通过六聚体内部和六聚体之间的 CA-CA 相互作用稳定，这对于调节病毒组装和成熟非常重要。我们应用子断层平均和分类来确定晶格边缘处 CA 的寡聚状态，发现 CA 形成部分六聚体。这些结构揭示了 CA-SP1 在晶格边缘形成的相互作用网络。我们还对稳定不成熟晶格和部分 CA-SP1 螺旋束的 CA-CA 相互作用进行了原子分子动力学模拟。自由能计算表明，与完整六螺旋束相比，部分六聚体中的螺旋到线圈转变的趋势增加。总之，这些结果表明 CA 二聚体是晶格组装的基本单位，部分六聚体存在于晶格边缘，它们处于螺旋-线圈动态平衡中，部分螺旋束更容易展开，代表 HIV-1 成熟起始的潜在部位。
