Transcriptional regulation is fundamental for the proper control of cellular processes. The Mediator is a transcriptional co-regulator complex that is required for the expression of most genes, yet some Mediator subunits selectively affect biological processes. In C. elegans, the Mediator subunit MDT-15 is required for the transcription of fatty acid metabolism genes. Fatty acids are common components of most lipids, and individual fatty acids profoundly impact membrane properties and other cellular functions. In this study, we find that inactivating
mdt-15 severely disrupts endoplasmic reticulum (ER) homeostasis. In particular, depletion or mutation of
mdt-15 results in an overall shift from unsaturated to saturated fatty acids in most lipid species, although the overall abundance of individual lipid species is not altered. The most significant changes in fatty acid distribution occur in phosphatidylcholine, a major membrane phospholipid. The altered membrane lipid fatty acid compositions in
mdt-15 worms is accompanied by severe ER stress as judged by upregulation of the ER chaperone
hsp-4/BiP, the spliced RNA encoding the ER stress effector XBP-1, and the phosphorylation of the translation initiation factor eIF2a. Surprisingly, dietary supplementation of
mdt-15 worms with unsaturated fatty acids only partially alleviates the chronic ER stress, suggesting that
mdt-15 maintains ER homeostasis by controlling membrane lipid fatty acid components as well as other unidentified processes. We test whether
mdt-15 is directly involved in the unfolded protein response, a conserved feedback mechanism that monitors ER proteostatic status. We show that inactivation of
mdt-15 does not render worms hypersensitivity to ER stress or impair the induction of unfolded protein response genes. On the contrary, inactivation of
mdt-15 constitutively activates the unfolded protein response. Together, we demonstrate that
mdt-15 is required for ER homeostasis, specifically through regulation of proper membrane lipid unsaturation and potentially other unidentified processes.