Updated project metadata. The acetyl-CoA transporter, AT-1 (also referred to as SLC33A1), is a key member of the endoplasmic reticulum (ER) acetylation machinery; it transports acetyl-CoA from the cytosol into the ER lumen where it serves as donor of the acetyl group for Nε-lysine acetylation 1,2. Dysfunctional ER acetylation, as caused by heterozygous or homozygous mutations as well as gene duplication events of AT-1/SLC33A1, has been linked to both developmental and age-associated human diseases 3-7. Mice with reduced or increased AT-1 expression mimic associated human diseases 8-10. In this study, we investigated the pervasive effects that dysregulated AT-1 has on intracellular acetyl-CoA homeostasis. Specifically, we used AT-1S113R/+ mice 8, a model of AT-1 haploinsufficiency, and AT-1 sTg mice 10, a model of AT-1 overexpression. We found that reduced AT-1 activity in AT-1S113R/+ mice led to increased availability of acetyl-CoA in the cytosol and spontaneous steatosis. Conversely, increased AT-1 activity decreased the availability of acetyl-CoA in the cytosol and made the animals resistant to diet-induced steatosis. Both models displayed significant metabolic adaptation involving different cellular organelles and compartments. Mechanistically, the metabolic adaptation was driven by changes in both protein levels (proteome) and stoichiometry of acetylation (acetylome) within fundamental pathways. Collectively, our results suggest that AT-1 acts as an important metabolic regulator that maintains acetyl-CoA homeostasis by promoting functional “cross-talk” between different intracellular organelles and compartments.