Molecular mechanism of CtBP1-S/BARS-driven membrane fission and its cellular control by metabolic ligands.

CtBP1-S/BARS (C-terminal binding protein 1-S/Brefeldin A ADP-Ribosylation Substrate) is a moonlighting protein with key roles in membrane trafficking and gene regulation. We show that CtBP1-S/BARS couples enzymatic lipid remodeling [lysophosphatidic acid (LPA)-to-phosphatidic acid (PA) conversion] with membrane deformation to drive fission and that this activity is directly controlled by metabolic ligands. CtBP1-S/BARS exists as a monomer or dimer. The monomeric, acyl-CoA-bound form drives membrane fission by coupling acyltransferase-dependent LPA-to-PA conversion with amphipathic helix insertion into PA-enriched membranes. Under metabolic stress, reduced nicotinamide adenine dinucleotide (NADH) binding triggers dimerization and structural rearrangements that disable fission, enabling the binding of transcription factors regulating apoptosis and energy metabolism. This NADH/acyl-CoA competition likely coordinates trafficking shutdown with gene expression programs through a single conformational change. In intact cells, increased NADH promotes nuclear accumulation of dimeric/tetrameric CtBP1-S/BARS, whereas elevated acyl-CoA favors the cytosolic, membrane-associated monomer, indicating that cofactor availability determines protein function under physiological and stress conditions. This work reveals the structural basis for integrating membrane transport with transcriptional control, demonstrating how evolution embeds distinct cellular functions into a unified molecular pathway.