7SK non-coding RNA (7SK) negatively regulates RNA polymerase II (RNA Pol II) elongation by inhibiting positive transcription elongation factor b (P-TEFb), and its ribonucleoprotein complex (RNP) is hijacked by HIV-1 for viral transcription and replication. Methylphosphate capping enzyme (MePCE) and La-related protein 7 (Larp7) constitutively associate with 7SK to form a core RNP, while P-TEFb and other proteins dynamically assemble to form different complexes. Here, we present the cryo-EM structures of 7SK core RNP formed with two 7SK conformations, circular and linear, and uncover a common RNA-dependent MePCE-Larp7 complex. Together with NMR, biochemical, and cellular data, these structures reveal the mechanism of MePCE catalytic inactivation in the core RNP, unexpected interactions between Larp7 and RNA that facilitate a role as an RNP chaperone, and that MePCE-7SK-Larp7 core RNP serves as a scaffold for switching between different 7SK conformations essential for RNP assembly and regulation of P-TEFb sequestration and release.

7SK非编码RNA（7SK）通过抑制正转录延伸因子b（P-TEFb）负向调节RNA聚合酶II（RNA Pol II）延伸，其核糖核蛋白复合物（RNP）被HIV-1劫持进行病毒转录和复制。甲基磷酸加帽酶 (MePCE) 和 La 相关蛋白 7 (Larp7) 与 7SK 组成型结合形成核心 RNP，而 P-TEFb 和其他蛋白动态组装形成不同的复合物。在这里，我们展示了由两种 7SK 构象（圆形和线性）形成的 7SK 核心 RNP 的冷冻电镜结构，并揭示了常见的 RNA 依赖性 MePCE-Larp7 复合物。结合 NMR、生化和细胞数据，这些结构揭示了核心 RNP 中 MePCE 催化失活的机制、Larp7 和 RNA 之间意外的相互作用促进了 RNP 伴侣的作用，以及 MePCE-7SK-Larp7 核心 RNP 充当用于在不同 7SK 构象之间切换的支架，这对于 RNP 组装以及 P-TEFb 封存和释放的调节至关重要。