mTORC1-independent translation control in mammalian cells by methionine adenosyltransferase 2A and S-adenosylmethionine

Methionine adenosyltransferase (MAT) catalyzes the synthesis of S-adenosylmethionine (SAM). As the sole methyl-donor of methylation of DNA, RNA and proteins, SAM amount affects gene expression by changing their methylation. Expression of MAT2A, the catalytic subunit of isozyme MAT2, is positively correlated with proliferation of cancer cells. However, how MAT2A promotes cell proliferation is largely unknown. Given that the protein synthesis is induced in proliferating cells and that RNA and protein components of translation machinery are methylated, we tested whether MAT2 and SAM are coupled with protein synthesis. By measuring ongoing protein translation with puromycin labeling in HeLa and Hepa1 cells, we revealed that MAT2A depletion by siRNA or chemical inhibition by cycloleucine reduced protein synthesis in mammalian cells. Overexpression of MAT2A enhanced protein synthesis, indicating that SAM is limiting under normal culture conditions. MAT2 inhibition did not accompany a reduction in the mTORC1 activity but reduced polysome formation. Polysome-bound RNA sequencing revealed that MAT2 inhibition decreased translation efficiency of a fraction of mRNAs. MAT2A was also found to interact with the proteins involved in rRNA processing and ribosome biogenesis. Depletion or inhibition of MAT2 reduced 18S rRNA processing. In addition, by quantitative mass spectrometry, we observed that some translation factors were dynamically methylated in response to the activity of MAT2A or the availability of SAM. These observations suggest that cells possess an mTOR-independent regulatory mechanism that tunes translation in response to the amount of SAM. Such a system may acclimate cells for survival when SAM synthesis is reduced whereas it may support proliferation when SAM is sufficient. Overall design: RNA sequencing of total 8 samples was performed. Each sample is biologically duplicated. There are four different conditions: CLEU-treated total RNA, CLEU-treated polysome-binding RNA, untreated total RNA, and untreated polysome-binding RNA.