Giant viruses encode vitamin K–based redox modules for lipid modification

Abstract Viruses with large DNA genomes often carry auxiliary metabolic genes that reprogram host physiology, yet their contributions to host redox and membrane homeostasis remains poorly understood. Here we report the discovery and functional reconstitution of viral homologs of vitamin K epoxide reductase (VKOR) encoded by giant viruses. Using phylogenetic and genomic context analysis, we find that viral VKOR genes are frequently located adjacent to γ-carboxylase-like epoxidase and fatty acid desaturase domains, consistent with a putative modular redox pathway for membrane lipid modification. To investigate their function, we expressed viral VKORs in an Escherichia coli strain lacking disulfide bond–forming enzymes and examined both their membrane topology and activity. Remarkably, a minimal set of residue substitutions enabled proper membrane insertion and restored bacterial motility, demonstrating that viral VKORs are catalytically competent electron shuttles. Structural modeling supports their integration into the endoplasmic reticulum–like environment in the host. Finally, we show that VKORs and γ-carboxylase-like epoxidase-desaturases from Fadolivirus and Yasminevirus giant viruses are expressed during infection of Vermamoeba vermiformis , where they may couple vitamin K epoxidation to desaturation-driven lipid remodeling. These findings expand the known functional repertoire of giant viruses and uncover a previously unrecognized viral strategy for manipulating host redox metabolism and membrane composition. Significance statement Viruses with large DNA genomes often encode metabolic enzymes that reshape host physiology, yet their ability to control host redox and membrane composition has remained unclear. We discovered that giant viruses encode a vitamin K epoxide reductase and associated enzymes that together form a putative redox module for lipid remodeling. We reconstituted viral VKOR activity in E. coli , demonstrating that these enzymes are catalytically active. We also detected their expression, at the RNA and protein levels, during amoebal infection by two giant viruses. Our findings reveal a previously unrecognized viral strategy for coupling vitamin K redox cycling to fatty acid desaturation. This work broadens our understanding of how giant viruses manipulate host redox homeostasis and membrane architecture during infection.