Dynamic m6A mRNA methylation directs translational control of heat shock response

Under stress, such as heat shock, the N6-methyladenosine (m6A) modification is shown to accumulate primarily in the 5′ untranslated region of induced mRNAs owing to the translocation of an m6A interacting protein, YTHDF2, into the nucleus, resulting in increased cap-independent translation of these mRNAs, indicating one possible mechanism by which stress-responsive genes can be preferentially expressed. The distribution of the N6-methyladenosine (m6A) base modification along mRNAs is uneven — it is under-represented in the 5′ untranslated region (5′ UTR) compared to other regions. Shu-Bing Qian and colleagues have found that under a stress such as heat shock, the m6A modification accumulates primarily in the 5′ UTR of induced mRNAs due to the translocation of an m6A interacting protein, YTHDF2, into the nucleus. YTHDF2 blocks the action of the m6A 'eraser' protein FTO, resulting in increased cap-independent translation of these mRNAs. The results indicate one possible mechanism by which stress-responsive genes can be preferentially expressed. The most abundant mRNA post-transcriptional modification is N6-methyladenosine (m6A), which has broad roles in RNA biology1,2,3,4,5. In mammalian cells, the asymmetric distribution of m6A along mRNAs results in relatively less methylation in the 5′ untranslated region (5′UTR) compared to other regions6,7. However, whether and how 5′UTR methylation is regulated is poorly understood. Despite the crucial role of the 5′UTR in translation initiation, very little is known about whether m6A modification influences mRNA translation. Here we show that in response to heat shock stress, certain adenosines within the 5′UTR of newly transcribed mRNAs are preferentially methylated. We find that the dynamic 5′UTR methylation is a result of stress-induced nuclear localization of YTHDF2, a well-characterized m6A ‘reader’. Upon heat shock stress, the nuclear YTHDF2 preserves 5′UTR methylation of stress-induced transcripts by limiting the m6A ‘eraser’ FTO from demethylation. Remarkably, the increased 5′UTR methylation in the form of m6A promotes cap-independent translation initiation, providing a mechanism for selective mRNA translation under heat shock stress. Using Hsp70 mRNA as an example, we demonstrate that a single m6A modification site in the 5′UTR enables translation initiation independent of the 5′ end N7-methylguanosine cap. The elucidation of the dynamic features of 5′UTR methylation and its critical role in cap-independent translation not only expands the breadth of physiological roles of m6A, but also uncovers a previously unappreciated translational control mechanism in heat shock response.