The YTHDF protein family, which contains three mammalian members (YTHDF1–YTHDF3), recognizes N6-methyladenosine (m6A) in RNA and mediates m6A-related biological functions, such as RNA degradation and translation. However, there are contradictory reports about the cellular roles of YTHDF isotypes and their regulatory mechanisms are unclear. For example, YTHDF1 and YTHDF3 (YTHDF1/3) proteins have been shown to promote translation in HeLa cells, but similar effects were not observed in HEK293T cells. Chen et al. have now shown that YTHDF1/3 proteins are subject to O-GlcNAcylation, a regulatory post-translational modification, which may account for their cell type-specific translation regulatory effects.
O-GlcNAcylation was identified on YTHDF1/3, but not on YTHDF2. The O-GlcNAc level on YTHDF1/3 is cell type-dependent, as HEK293T cells have higher levels of YTHDF1/3 O-GlcNAcylation than HeLa cells, which correlates with the higher expression of O-GlcNAc transferase — the enzyme that catalyzes O-GlcNAcylation — in HEK293T cells. The high O-GlcNAc level suppresses the translation promotion function of YTHDF1/3 in HEK293T cells, which explains the lack of translation effects of YTHDF1/3 in this cell line. The team identified the site modified by O-GlcNAc and engineered YTHDF1/3 mutants with alanine substitutions at these sites. Comparing the interactome of the wild-type and mutant YTHDF1/3 proteins showed that unmodified YTHDF1/3 tended to associate with translation-related proteins, whereas O-GlcNAcylation blocked such interactions. Other work has suggested that the YTHDF1-binding protein FMRP could sequester YTHDF1 and prevent its translation-enhancing function in neurons, which provides an alternative explanation for the cell-type specific effect.
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