Terminal oligopyrimidine motif-containing mRNAs (TOPs) encode all ribosomal proteins in mammals and are regulated to tune ribosome synthesis to cell state. Previous studies have implicated LARP1 in 40S- or 80S-ribosome complexes that are thought to repress and stabilize TOPs. However, a molecular understanding of how LARP1 and TOPs interact with these ribosome complexes is lacking. Here, we show that LARP1 directly binds non-translating ribosomal subunits. Cryo-EM structures reveal a previously uncharacterized domain of LARP1 bound to and occluding the mRNA channel of the 40S subunit. Increased availability of free ribosomal subunits downstream of various stresses promote 60S joining at the same interface to form LARP1-80S complexes. Simultaneously, LARP1 engages the TOP via its previously characterized La/PAM2 and DM15 domains. Contrary to expectations, ribosome binding within these complexes is not required for LARP1-mediated TOP repression or stabilization, two canonical LARP1 functions. Together, this work provides molecular insight into how LARP1 directly binds ribosomal subunits and challenges existing models describing the function of repressed LARP1-40S/80S-TOP complexes.

中文翻译：

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LARP1 结合抑制复合物中的核糖体和 TOP mRNA。


含有末端寡嘧啶基序的 mRNA （TOPs） 编码哺乳动物中的所有核糖体蛋白，并受到调节以将核糖体合成调整到细胞状态。先前的研究表明 LARP1 与 40S 或 80S 核糖体复合物有关，这些复合物被认为可以抑制和稳定 TOPs。然而，缺乏对 LARP1 和 TOP 如何与这些核糖体复合物相互作用的分子理解。在这里，我们表明 LARP1 直接结合非翻译核糖体亚基。冷冻电镜结构揭示了 LARP1 的一个以前未表征的结构域，该结构域与 40S 亚基的 mRNA 通道结合并遮挡。各种应力下游游离核糖体亚基的可用性增加，促进 60S 在同一界面连接形成 LARP1-80S 复合物。同时，LARP1 通过其先前表征的 La/PAM2 和 DM15 结构域与 TOP 结合。与预期相反，这些复合物内的核糖体结合不是 LARP1 介导的 TOP 抑制或稳定所必需的，这是 LARP1 的两种经典功能。总之，这项工作提供了关于 LARP1 如何直接结合核糖体亚基的分子见解，并挑战了描述被抑制的 LARP1-40S/80S-TOP 复合物功能的现有模型。