Regulation of Unsaturated Fatty Acid Biosynthesis in Saccharomyces

Abstract

The Saccharomyces cerevisiae OLE1 gene encodes a membrane-bound ⌬9 fatty-acid desaturase, whose expression is regulated through transcriptional and mRNA stability controls. In wild type cells grown on fatty acid-free medium, OLE1 mRNA has a half-life of 10 ؎ 1.5 min (basal stability) that becomes highly unstable when cells are exposed to unsaturated fatty acids (regulated stability). Activation of OLE1 transcription is dependent on N-terminal fragments of two membrane proteins, Mga2p and Spt23p, that are proteolytically released from the membrane by a ubiquitin-mediated mechanism. Surprisingly, disruption of the MGA2 gene also reduces the half-life of the OLE1 transcript and abolishes fatty acid regulated instability. Disruption of its cognate, SPT23, has no effect on the half-life of the mRNA. Mga2p appears to have two distinct functions with respect to the OLE1 mRNA stability: a stabilizing effect in cells grown in fatty acid-free medium and a destabilizing function in cells that are exposed to unsaturated fatty acids. These functions are independent of OLE1 transcription and can confer basal and regulated stability on OLE1 mRNAs that are produced under the control of the unrelated GAL1 promoter. Expression of soluble, N-terminal fragments of Mga2p stabilize the transcript but do not confer fatty acid-regulated instability on the mRNA suggesting that the stabilizing functions of Mga2p do not require membrane processing and that modifications to the protein introduced during proteolysis may play a role in the destabilizing effect. An analysis of mutants that are defective in mRNA degradation indicate that the Mga2p-requiring control mechanism that regulates the fatty acid-mediated instability of the OLE1 transcript acts by activating exosomal 3 3 5-exonuclease degradation activity.Unsaturated fatty acids are formed in the yeast Saccharomyces cerevisiae by Ole1p, a membrane-bound ⌬9 fatty-acid desaturase that converts long chain saturated fatty acyl-CoA substrates into monounsaturated species (1, 2). Expression of OLE11 is regulated by nutrient fatty acids by both transcriptional and mRNA stability controls. Recently, the expression of Ole1p was found to be dependent on two homologous ER membrane proteins, Mga2p and Spt23p (3). These are cleaved from the membrane by a novel ubiquitin-mediated proteolytic mechanism that releases soluble N-terminal polypeptides (4). Although neither protein appears to have a DNA binding domain, the N-terminal fragments specifically activate OLE1 transcription presumably by influencing chromatin structure or by associating with DNA binding proteins that are bound to elements on the OLE1 promoter (5, 6). Disruption of either SPT23 or MGA2 does not affect the growth rate or production of fatty acids under normal laboratory growth conditions (7). Disruption of both genes, however, blocks expression of Ole1p and results in a synthetic auxotrophy that can be overcome by adding unsaturated fatty acids to the medium or expressing OLE1 under a different promoter (4). Evidence for the direc...

A Lipid E-MAP Identifies Ubx2 as a Critical Regulator of Lipid Saturation and Lipid Bilayer Stress

“…Specifically, we found a strong negative genetic interaction between the UBX2 gene, which encodes a component of the ERAD pathway (Neuber et al, 2005), and OLE1 , the single and essential FA desaturase gene of S. cerevisiae (Figures 2D–2F). In the same network of genetic interactions, we also found MGA2 and SPT23 , which encode two transcriptional regulators that cooperatively mediate and modulate the expression of OLE1 (Figure 2F) (Kandasamy et al, 2004). These regulators are ER membrane proteins and have to be activated in order to drive OLE1 transcription (Figure 2G).…”

“…As a feedback control, excess dietary UFAs abrogate the expression of OLE1 and destabilize the mRNA of OLE1 (Kandasamy et al, 2004). In the presence of dietary linoleate (18:2), the OLE1 mRNA level dropped significantly in WT and ubx2 Δ, mga2 Δ, and spt23 Δ cells (Figure 4A).…”

An ubiquitin-dependent balance between mitofusin turnover and fatty acids desaturation regulates mitochondrial fusion

“…8a and ref. 43). These marked differences in function and fusion between ubp2Δ and ubp2Δ mga2Δ cells were observed despite the levels of mitofusins being similarly low whether or not the FZO1 extra copy was expressed (Fig.…”

Multiple Endoplasmic Reticulum-to-Nucleus Signaling Pathways Coordinate Phospholipid Metabolism with Gene Expression by Distinct Mechanisms

“…Although the compact clustering of these genes suggested that their expression may respond to a common signal, comparison to the genome-wide location analysis data set from Harbison et al (30) failed to reveal enrichment for any specific transcription factors. However, we noted the presence of OLE1, a gene whose transcriptional regulation has been intensively studied (22,23,51,(85)(86)(87)(88), in supercluster V. OLE1 expression is regulated by two homologous transcription factors, Mga2p and Spt23p (89,90). The inactive forms of both transcription factors are anchored in the ER through a C-terminal transmembrane domain and are activated by proteolytic cleavage from the membrane through a ubiquitin-mediated mechanism that involves processing by the 26 S proteasome (91)(92)(93).…”