PUB - Publikationen an der Universität Bielefeld

Gene regulation dependent on N6-methyladenosine (m6A) in mRNA involves RNA-binding proteins that recognize m6A through a YTH domain. The Arabidopsis YTH-domain protein ECT2 is thought to influence mRNA 3′-end formation via binding to URU(m6A)Y sites, an unexpected conclusion given that ECT2 functions require its m6A binding activity, and that RR(m6A)CH is the m6A consensus site in all eukaryotes. Here, we apply the orthogonal techniques individual nucleotide-resolution UV-crosslinking and immunoprecipitation (iCLIP) and HyperTRIBE to define high-quality target sets of the YTH-domain proteins ECT2 and ECT3. The results show that in vivo, ECT2 does in fact bind to RR(m6A)CH. URUAY and other pyrimidine-rich motifs are enriched around, but not at m6A-sites, reflecting a preference for N6-adenosine methylation of RRACH islands in pyrimidine-rich regions. Such regions may also be implicated in ECT2-binding. In particular, a series of properties unique to the URUAY motif suggest that URUAY-type sequences act as sites of competition between unknown RNA-binding proteins and the intrinsically disordered region of ECT2. We also show that the abundance of many ECT2/3 mRNA targets is decreased in meristematic cells devoid of ECT2/3/4-activity. In contrast, loss of ECT2/3/4 activity has no effect on polyadenylation site usage in ECT2/3 targets, consistent with the exclusive cytoplasmic localization of ECT2 observed by super-resolution confocal microscopy. Our study reconciles conflicting results between genetic observations on N6-adenosine methylation and ECT2/3/4 function on the one side, and ECT2 target identification on the other, and point to regulation of cytoplasmic mRNA function, including abundance, as a mechanism of plant YTHDF action.