Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA.
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
EMBO J. 2018 Aug 15;37(16). doi: 10.15252/embj.201899264. Epub 2018 Jul 4.
Membrane proteins are aggregation-prone in aqueous environments, and their biogenesis poses acute challenges to cellular protein homeostasis. How the chaperone network effectively protects integral membrane proteins during their post-translational targeting is not well understood. Here, biochemical reconstitutions showed that the yeast cytosolic Hsp70 is responsible for capturing newly synthesized tail-anchored membrane proteins (TAs) in the soluble form. Moreover, direct interaction of Hsp70 with the cochaperone Sgt2 initiates a sequential series of TA relays to the dedicated TA targeting factor Get3. In contrast to direct loading of TAs to downstream chaperones, stepwise substrate loading via Hsp70 maintains the solubility and targeting competence of TAs, ensuring their efficient delivery to the endoplasmic reticulum (ER). Inactivation of cytosolic Hsp70 severely impairs TA translocation Our results demonstrate a new role of cytosolic Hsp70 in directly assisting the targeting of an essential class of integral membrane proteins and provide a paradigm for how "substrate funneling" through a chaperone cascade preserves the conformational quality of nascent membrane proteins during their biogenesis.
膜蛋白在水相环境中易于聚集，其生物发生对细胞蛋白质的内稳态构成了严峻的挑战。伴侣网络如何在翻译后靶向过程中有效地保护完整的膜蛋白，目前还不是很清楚。在这里，生化重构表明酵母胞质 Hsp70 负责以可溶性形式捕获新合成的尾部锚定膜蛋白（TA）。此外，Hsp70 与伴侣蛋白 Sgt2 的直接相互作用启动了一系列 TA 接力，将 TA 靶向到专门的 TA 靶向因子 Get3。与直接将 TA 加载到下游伴侣蛋白不同，通过 Hsp70 逐步加载底物可以保持 TA 的可溶性和靶向能力，确保它们有效地递送到内质网（ER）。胞质 Hsp70 的失活严重损害了 TA 的易位。我们的结果表明，胞质 Hsp70 在直接辅助一类重要的完整膜蛋白的靶向中具有新的作用，并为通过伴侣蛋白级联进行“底物通道化”如何在其生物发生过程中保持新生膜蛋白的构象质量提供了范例。
