Dynamic m ⁶A mRNA methylation directs translational control of heat shock response

Abstract

The most abundant mRNA post-transcriptional modification is N ⁶-methyladenosine (m ⁶A) that has broad roles in RNA biology ^{1- 5} . In mammalian cells, the asymmetric distribution of m ⁶A along mRNAs leaves relatively less methylation in the 5′ untranslated region (5′UTR) compared to other regions ^{6, 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 whether m ⁶A modification influences mRNA translation. Here we show that in response to heat shock stress, m ⁶A is preferentially deposited to the 5′UTR of newly transcribed mRNAs. We found that the dynamic 5′UTR methylation is a result of stress-induced nuclear localization of YTHDF2, a well characterized m ⁶A “reader”. Upon heat shock stress, the nuclear YTHDF2 preserves 5′UTR methylation of stress-induced transcripts by limiting the m ⁶A “eraser” FTO from demethylation. Remarkably, the increased 5′UTR methylation in the form of m ⁶A 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 site m ⁶A modification in the 5′UTR enables translation initiation independent of the 5′ end m ⁷G cap. The elucidation of the dynamic feature of 5′UTR methylation and its critical role in cap-independent translation not only expands the breadth of physiological roles of m ⁶A, but also uncovers a previously unappreciated translational control mechanism in heat shock response.