Ashbya gossypii as a model system to study septin organization by single-molecule localization microscopy

Reconstituted in vitro systems to reveal the roles and functions of septins

Septins are essential cytoskeletal proteins involved in key cellular processes and have also been implicated in diseases from cancers to neurodegenerative pathologies. However, they have not been as thoroughly studied as other cytoskeletal proteins. In vivo, septins interact with other cytoskeletal proteins and with the inner plasma membrane. Hence, bottom-up in vitro cell-free assays are well suited to dissect the roles and behavior of septins in a controlled environment. Specifically, in vitro studies have been invaluable in describing the self-assembly of septins into a large diversity of ultrastructures. Given that septins interact specifically with membrane, the details of these septin-membrane interactions have been analyzed using reconstituted lipid systems. In particular, at a membrane, septins are often localized at curvatures of micrometer scale. In that context, in vitro assays have been performed with substrates of varying curvatures (spheres, cylinders or undulated substrates) to probe the sensitivity of septins to membrane curvature. This Review will first present the structural properties of septins in solution and describe the interplay of septins with cytoskeletal partners. We will then discuss how septins interact with biomimetic membranes and induce their reshaping. Finally, we will highlight the curvature sensitivity of septins and how they alter the mechanical properties of membranes.

Sporulation in Ashbya gossypii

Ashbya gossypii is a filamentous ascomycete belonging to the yeast family of Saccharomycetaceae. At the end of its growth phase Ashbya generates abundant amounts of riboflavin and spores that form within sporangia derived from fragmented cellular compartments of hyphae. The length of spores differs within species of the genus. Needle-shaped Ashbya spores aggregate via terminal filaments. A. gossypii is a homothallic fungus which may possess a and α mating types. However, the solo-MATa type strain is self-fertile and sporulates abundantly apparently without the need of prior mating. The central components required for the regulation of sporulation, encoded by IME1, IME2, IME4, KAR4, are conserved with Saccharomyces cerevisiae. Nutrient depletion generates a strong positive signal for sporulation via the cAMP-PKA pathway and SOK2, which is also essential for sporulation. Strong inhibitors of sporulation besides mutations in the central regulatory genes are the addition of exogenous cAMP or the overexpression of the mating type gene MATα2. Sporulation has been dissected using gene-function analyses and global RNA-seq transcriptomics. This revealed a role of Msn2/4, another potential PKA-target, for spore wall formation and a key dual role of the protein A kinase Tpk2 at the onset of sporulation as well as for breaking the dormancy of spores to initiate germination. Recent work has provided an overview of ascus development, regulation of sporulation and spore maturation. This will be summarized in the current review with a focus on the central regulatory genes. Current research and open questions will also be discussed.