Abstract
N6 methylation at adenosine 1832 (m6A1832) of mammalian 18S rRNA, occupying a critical position within the decoding center, is modified by a conserved methyltransferase, METTL5. Here, we find that METTL5 shows strong substrate preference toward the 18S A1832 motif but not the other reported m6A motifs. Comparison with a yeast ribosome structural model unmodified at this site indicates that the modification may facilitate mRNA binding by inducing conformation changes in the mammalian ribosomal decoding center. METTL5 promotes p70-S6K activation and proper translation initiation, and the loss of METTL5 significantly reduces the abundance of polysome. METTL5 expression is elevated in breast cancer patient samples and is required for growth of several breast cancer cell lines. We further find that Caenorhabditis elegans lacking the homolog metl-5 develop phenotypes known to be associated with impaired translation. Altogether, our findings uncover critical and conserved roles of METTL5 in the regulation of translation.
Highlights
Mammalian ribosome is composed of two functional components, the small subunit (40S) and the large subunit (60S), including nearly 80 ribosomal proteins (RPs) and four ribosomal RNAs, which are orchestrated by translation factors and transfer RNAs to control the fidelity and rate of translation
After high-performance liquid chromatography-mass spectrometry (HPLC-MS) analyses, we found that METTL5 showed specific activities toward the UAACA motif, which is derived from 18S ribosomal RNAs (rRNAs) A1832 site, but not the 28S rRNA m6A motif UAACG containing the A4220 site and the METTL3/METTL14-preferred substrate motif RRACH (Figure 1A)
We investigated the requirement of the flanking sequences to the METTL5-mediated m6A catalysis, using synthetic RNA probes ranging from 5 to 19 nt
Summary
Mammalian ribosome is composed of two functional components, the small subunit (40S) and the large subunit (60S), including nearly 80 ribosomal proteins (RPs) and four ribosomal RNAs (rRNAs), which are orchestrated by translation factors and transfer RNAs (tRNAs) to control the fidelity and rate of translation. J1056 and m7G1639 of 18S rRNA are conserved from yeasts to humans but are absent in prokaryotes. The m6A modification of 18S rRNA was initially reported in Xenopus laevis (A1789) and human (A1832) (Choi and Busch, 1978; Maden, 1986), and recently in C. elegans (A1717) (Liberman et al, 2020) as well as other metazoans and archaea (Chen et al, 2020; Coureux et al, 2020; Ignatova et al, 2020; Leismann et al, 2020; Natchiar et al, 2017; Nu€renberg-Goloub et al, 2020; van Tran et al, 2019; Xing et al, 2020), but was not identified in prokaryotes and yeasts (Sergiev et al, 2018). On the basis of these observations, it is plausible that certain regulatory functions, rather than housekeeping roles, may be carried by these variable modifications to alter cellular translation activities, in response to capricious environmental pressures
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