The OLE pathway of yeast regulates the abundance of the ER‐bound enzyme Δ‐9 fatty acid desaturase OLE1, thereby controlling unsaturated fatty acid pools and membrane fluidity. Previously, we showed that this pathway is exquisitely regulated by the ubiquitin/proteasome system. Activation of the pathway involves proteasomal processing of a membrane‐bound transcription factor and the subsequent mobilization of the cleaved, ubiquitylated transcription factor from its partner molecule by CDC48^UFD1/NPL4, a ubiquitin‐selective chaperone‐like enzyme. Here we report that the OLE1 protein itself is naturally short‐lived and is degraded by ubiquitin/proteasome‐dependent ER‐associated degradation (ERAD). We found that CDC48^UFD1/NPL4 plays a second role in the OLE pathway by mediating ERAD of OLE1. Intriguingly, other ERAD substrates also require CDC48^UFD1/NPL4 for degradation, indicating that this enzyme is a novel, constitutive component of the ERAD machinery. We propose that CDC48^UFD1/NPL4 functions as a segregase that liberates ubiquitylated proteins from non‐modified partners.

Transcriptionally silent chromatin localizes to the nuclear periphery, which provides a special microenvironment for gene repression. A variety of nuclear membrane proteins interact with repressed chromatin, yet the functional role of these interactions remains poorly understood. Here, we show that, in Schizosaccharomyces pombe, the nuclear membrane protein Lem2 associates with chromatin and mediates silencing and heterochromatin localization. Unexpectedly, we found that these functions can be separated and assigned to different structural domains within Lem2, excluding a simple tethering mechanism. Chromatin association and tethering of centromeres to the periphery are mediated by the N-terminal LEM (LAP2–Emerin–MAN1) domain of Lem2, whereas telomere anchoring and heterochromatin silencing require exclusively its conserved C-terminal MSC (MAN1–Src1 C-terminal) domain. Particularly, silencing by Lem2 is epistatic with the Snf2/HDAC (histone deacetylase) repressor complex SHREC at telomeres, while its necessity can be bypassed by deleting Epe1, a JmjC protein with anti-silencing activity. Furthermore, we found that loss of Lem2 reduces heterochromatin association of SHREC, which is accompanied by increased binding of Epe1. This reveals a critical function of Lem2 in coordinating these antagonistic factors at heterochromatin. The distinct silencing and localization functions mediated by Lem2 suggest that these conserved LEM-containing proteins go beyond simple tethering to play active roles in perinuclear silencing.

UV irradiation stimulates expression of the gene encoding the key enzyme chalcone synthase (CHS), which leads to the generation of protective flavonoids in parsley cell cultures. CHS transcripts increase after 3 to 4 hr, and early genes are involved in the signal transduction to the CHS promoter. By using the fluorescent differential display technique in a large-scale screening, several early UV light-induced genes were isolated. Of these, a novel glutathione S-transferase (PcGST1) is induced within 2 hr and precedes CHS expression. Overexpression of PcGST1 in transformed cell lines containing a CHS promoter/luciferase reporter (CHS-LUC) affected the onset of LUC transcription. Supplementing these cell lines with glutathione immediately stimulated CHS-LUC expression within 2 hr in dark-incubated cells and resulted in a biphasic induction profile in UV-irradiated cells. Our data indicate the involvement of glutathione and PcGST1 in early events of a UV light-dependent signal transduction pathway to CHS. In this context, the oxidative status of a cell acts as a central regulating element.