Glucose starvation triggers filamentous septin assemblies in an S. pombe septin-2 deletion mutant

• Published in: Biology open Vol. 8; no. 1
• Main Authors: Liu, Minghua, Heimlicher, Maria B, Bächler, Mirjam, Ibeneche-Nnewihe, Chieze C, Florin, Ernst-Ludwig, Brunner, Damian, Hoenger, Andreas
• Format: Journal Article
• Language: English
• Published: England The Company of Biologists Ltd 01.01.2019; The Company of Biologists
• Subjects: Actin; Assemblies; Clonal deletion; Conventional and cryo-electron microscopy; Correlative light and electron microscopy; Cytology; Cytoplasm; Deletion mutant; Electron micrographs; Electron microscopy; Filamentous septin assemblies; Filaments; Fission; Fluorescence; Fluorescence microscopy; Gene deletion; Glucose; Glucose starvation in S. pombe; Localization; Micrography; Microscopy; Mitochondria; Polymers; Proteins; Ribosomes; Septin; Septins; Vitrified sectioning; Yeast; Yeasts; Conventional and cryo-electron microscopy; Correlative light and electron microscopy; Vitrified sectioning; Filamentous septin assemblies; Glucose starvation in S. pombe; Septins
• ISSN: 2046-6390; 2046-6390
• DOI: 10.1242/bio.037622

Using correlative light and electron microscopy (CLEM), we studied the intracellular organization by of glucose-starved fission yeast cells ( ) with regards to the localization of septin proteins throughout the cytoplasm. Thereby, we found that for cells carrying a deletion of the gene encoding septin-2 (spn2Δ), starvation causes a GFP-tagged version of septin-3 (spn3-GFP) and family members, to assemble into a single, prominent filamentous structure. It was previously shown that during exponential growth, spn2Δ cells form septin-3 polymers. However, the polymers we observed during exponential growth are different from the spn3p-GFP structure we observed in starved cells. Using CLEM, in combination with anti-GFP immunolabeling on plastic-sections, we could assign spn3p-GFP to the filaments we have found in EM pictures. Besides septin-3, these filamentous assemblies most likely also contain septin-1 as an RFP-tagged version of this protein forms a very similar structure in starved spn2Δ cells. Our data correlate phase-contrast and fluorescence microscopy with electron micrographs of plastic-embedded cells, and further on with detailed views of tomographic 3D reconstructions. Cryo-electron microscopy of spn2Δ cells in vitrified sections revealed a very distinct overall morphology of the spn3p-GFP assembly. The fine-structured, regular density pattern suggests the presence of assembled septin-3 filaments that are clearly different from F-actin bundles. Furthermore, we found that starvation causes substantial mitochondria fission, together with massive decoration of their outer membrane by ribosomes.