Metabolite import via SLC33A1 enables ATF6 activation by endoplasmic reticulum stress
The transcription factor ATF6 has a central role in adapting mammalian cells to endoplasmic reticulum (ER) stress via the Unfolded Protein Response (UPR). This has driven efforts to identify modulators of ATF6 signalling. Here, an unbiased genome-wide CRISPR-Cas9 screen performed in Chinese Hamster Ovary (CHO) cells revealed that proteolytic processing of the ATF6 precursor to its active form was impaired in CHO cells lacking the ER-resident solute carrier SLC33A1, a transporter involved in acetyl-CoA import, sialylation and N{varepsilon}-lysine protein acetylation. Cells lacking SLC33A1 constitutively trafficked the ATF6 precursor to the Golgi, but exhibit impaired subsequent Golgi processing, correlating with altered ATF6 Golgi glycosylation. SLC33A1 deficiency also deregulated activation of the IRE1 branch of the UPR, pointing to a selective loss of ATF6-mediated negative feedback in the UPR. Notably, Slc33a1-deleted cells accumulated higher levels of unmodified sialylated N-glycans, precursors to acetylated glycans, likely reflecting impaired glycan processing. By contrast, deletion of ER-localised acetyltransferases NAT8 and NAT8B, which catalyse protein N{varepsilon}-lysine acetylation in the secretory pathway, did not replicate the ATF6 processing defects observed in Slc33a1-deficient cells. Together, our findings highlight a role for SLC33A1-mediated metabolite transport in the post-ER maturation of ATF6 and point direct links between small-molecule metabolism and branch-specific signalling in the UPR.