ALBA-YTHDF-m6A axis in plants. ALBA proteins facilitate cytoplasmic YTHDF-mediated reading of m6A in plants

N6-methyladenosine (m6A) in mRNA exerts many of its effects in regulation of eukaryotic gene expression by recruitment of cytoplasmic RNA-binding proteins containing a YT521-B homology (YTH) domain specialized for m6A-interaction. In plants, these so-called YTHDF proteins promote growth by stimulation of cell division in organ primordia by ill-defined mechanisms. Here, we show that the widely conserved family of mRNA-binding ALBA proteins interact with YTHDF proteins in the plant Arabidopsis thaliana. mRNA targets of ALBA proteins and the YTHDF proteins ECT2 and ECT3 overlap substantially, and nearly all ALBA4 targets with 3’-UTR binding sites are also m6A-ECT2/ECT3 targets. ALBA4 binding sites are located immediately upstream of m6A sites and correspond to the pyrimidine-rich elements previously suggested to be important for ECT2 binding. Accordingly, an ECT2 mutant protein defective in ALBA association binds less RNA in vivo compared to wild type, and many ECT2 targets exhibit less association with ECT2 in vivo in the absence of four of the six arabidopsis ALBA proteins. We propose that ALBA proteins facilitate assembly and function of m6A-YTHDF-containing mRNPs, thereby promoting plant growth.

信使RNA（mRNA）上的N6-甲基腺嘌呤（N6-methyladenosine, m6A）修饰，主要通过招募含有YT521-B同源（YT521-B homology, YTH）结构域、专性结合m6A的细胞质RNA结合蛋白，来介导其对真核基因表达的多数调控功能。在植物中，这类被命名为YTHDF的蛋白通过尚不明确的分子机制，促进器官原基的细胞分裂，进而推动植株生长。本研究证实，在模式植物拟南芥（Arabidopsis thaliana）中，广泛保守的mRNA结合蛋白ALBA家族可与YTHDF蛋白发生特异性相互作用。ALBA蛋白与YTHDF蛋白ECT2、ECT3的mRNA靶标存在大量重叠，且几乎所有带有3’非翻译区（3’-UTR）结合位点的ALBA4靶标，同时也属于m6A-ECT2/ECT3的靶标集合。ALBA4的结合位点紧邻m6A修饰位点，与此前被报道对ECT2结合至关重要的富嘧啶序列元件完全匹配。相应地，无法与ALBA结合的ECT2突变体在体内的RNA结合能力较野生型显著降低；而在拟南芥6个ALBA蛋白中缺失4个的突变体株系中，多数ECT2的靶标与ECT2的体内结合水平也出现明显下降。本研究提出，ALBA蛋白可促进含有m6A-YTHDF复合物的信使核糖核蛋白（mRNPs）的组装与功能行使，最终推动植物生长。