The Nuclear Envelope Proteins Esc1 and Mps3 Differentially Impact Sterol Gradients in Budding Yeast

Abstract

The metabolically stable lysolipid analogue edelfosine is an antitumor/antiparasitic drug proposed to act by disrupting lipid rafts and reducing the accessible sterol pool at the plasma membrane (PM). Once internalized, edelfosine also induces deformation of the nuclear envelope (NE) and disrupts telomere clustering in yeast. In this study we investigate the impact that NE-chromatin-anchoring pathways have on PM and NE/ER sterol homeostasis. Cells lacking Sir4 (sir4Δ) of the Silent Information Regulator histone deacetylase complex are resistant to edelfosine despite NE deformation induced by the drug. Using live fluorescence microscopy, we show herein that in sir4Δ yeast sterols remain accessible at the PM and redistribute from the PM of daughter cells to the PM of mother cells in response to edelfosine. Since Sir4 is the scaffold component of the SIR complex that mediates telomere anchoring to the NE, we questioned if its interactors at the NE, Esc1, and Mps3, could also impact sterol mobilization in response to edelfosine. Cells lacking Esc1 mimicked the phenotypes of sir4Δ yeast in response to edelfosine. Unlike sir4Δ and esc1Δ yeast, cells carrying a truncated Mps3 unable to bind Sir4, mps3Δ65-145, displayed aberrant NE morphology, intracellular sterol punctate and sensitivity to edelfosine. Furthermore, significative differences in squalene to sterol esters ratios between esc1Δ and mps3Δ65-145 mutants were found. Altogether these results support a differential contribution of Esc1 and Mps3 to sterol homeostasis and establishment of its intracellular gradient. The Sir4-Esc1 interaction sensitizes cells to lysolipid toxicity and sterol transport from the PM, while Mps3 has a stronger influence on silencing and sterol retention capacity at the PM.