Metabolite import by SLC33A1 is required for ATF6 activation during endoplasmic reticulum stress

The transcription factor ATF6α has a central role in adapting mammalian cells to ER stress via the unfolded protein response (UPR), prompting efforts to identify ATF6α modulators. Here, an unbiased genome-wide CRISPR-Cas9 screen performed in Chinese Hamster Ovary cells revealed that proteolytic processing of the ATF6α precursor to its active form was impaired in cells lacking the ER-resident solute carrier SLC33A1, a transporter previously implicated in acetyl-CoA import, sialylation, and Nε-lysine protein acetylation. Cells lacking SLC33A1 constitutively trafficked the ATF6α to the Golgi but exhibited impaired Golgi processing and activating proteolysis. IRE1α signalling was derepressed by SLC33A1 deficiency consistent with selective loss of ATF6α-mediated negative feedback in the UPR. Slc33a1 -deleted cells accumulated unmodified sialylated N-glycans, precursors to acetylated glycans, likely reflecting impaired glycan processing. Deletion of ER-localised acetyltransferases NAT8 and NAT8B, which catalyse protein Nε-lysine acetylation in the secretory pathway, did not replicate the ATF6α processing defects observed in Slc33a1 -deficient cells. Together, our findings highlight a role of SLC33A1-mediated metabolite transport in the post-ER ATF6α maturation, linking small-molecule metabolism to branch-specific signalling in the UPR.