Ribosome biogenesis, which takes place mainly in the nucleolus, involves coordinated expression of pre-ribosomal RNAs (pre-rRNAs) and ribosomal proteins, pre-rRNA processing, and subunit assembly with the aid of numerous assembly factors. Our previous study showed that the Arabidopsis thaliana protein arginine methyltransferase AtPRMT3 regulates pre-rRNA processing; however, the underlying molecular mechanism remains unknown. Here, we report that AtPRMT3 interacts with Ribosomal Protein S2 (RPS2), facilitating processing of the 90S/Small Subunit (SSU) processome and repressing nucleolar stress. We isolated an intragenic suppressor of atprmt3-2, which rescues the developmental defects of atprmt3-2 while produces a putative truncated AtPRMT3 protein bearing the entire N-terminus but lacking an intact enzymatic activity domain We further identified RPS2 as an interacting partner of AtPRMT3, and found that loss-of-function rps2a2b mutants were phenotypically reminiscent of atprmt3, showing pleiotropic developmental defects and aberrant pre-rRNA processing. RPS2B binds directly to pre-rRNAs in the nucleus, and such binding is enhanced in atprmt3-2. Consistently, multiple components of the 90S/SSU processome were more enriched by RPS2B in atprmt3-2, which accounts for early pre-rRNA processing defects and results in nucleolar stress. Collectively, our study uncovered a novel mechanism by which AtPRMT3 cooperates with RPS2B to facilitate the dynamic assembly/disassembly of the 90S/SSU processome during ribosome biogenesis and repress nucleolar stress.

核糖体生物合成主要发生在核仁中，涉及核糖体前体 RNA (pre-rRNAs) 和核糖体蛋白的协调表达、前体 rRNA 加工以及借助众多组装因子的亚基组装。我们之前的研究表明，拟南芥蛋白精氨酸甲基转移酶 AtPRMT3 调节 pre-rRNA 加工；然而，潜在的分子机制仍然未知。在这里，我们报告 AtPRMT3 与核糖体蛋白 S2 (RPS2) 相互作用，促进 90S/小亚基 (SSU) 加工组的处理并抑制核仁应激。我们分离了一个atprmt3-2的基因内抑制子，它挽救了atprmt3-2的发育缺陷。同时产生一个推定的截断 AtPRMT3 蛋白，它带有整个 N 端但缺乏完整的酶活性域我们进一步确定 RPS2 作为 AtPRMT3 的相互作用伙伴，并发现功能丧失的rps2a2b突变体在表型上让人联想到atprmt3，显示出多向性发育缺陷和异常的前 rRNA 加工。RPS2B 直接与细胞核中的前 rRNA 结合，这种结合在atprmt3-2 中得到增强。一致地，在atprmt3-2 中，RPS2B 更丰富了 90S/SSU 进程组的多个组成部分，这解释了早期的前 rRNA 加工缺陷并导致核仁压力。总的来说，我们的研究揭示了 AtPRMT3 与 RPS2B 合作以促进核糖体生物发生过程中 90S/SSU 加工组的动态组装/拆卸和抑制核仁应激的新机制。