Role of Atg8 in the regulation of vacuolar membrane invagination

Abstract

Cellular heat stress can cause damage, and significant changes, to a variety of cellular structures. When exposed to chronically high temperatures, yeast cells invaginate vacuolar membranes. In this study, we found that the expression of Atg8, an essential autophagy factor, is induced after chronic heat stress. In addition, without Atg8, vacuolar invaginations are induced conspicuously, beginning earlier and invaginating vacuoles more frequently after heat stress. Our results indicate that Atg8’s invagination-suppressing functions do not require Atg8 lipidation, in contrast with autophagy, which requires Atg8 lipidation. Genetic analyses of vps24 and vps23 further suggest that full ESCRT machinery is necessary to form vacuolar invaginations irrespective of Atg8. In contrast, through a combined mutation with the vacuole BAR domain protein Ivy1, vacuoles show constitutively enhanced invaginated structures. Finally, we found that the atg8Δivy1Δ mutant is sensitive against agents targeting functions of the vacuole and/or plasma membrane (cell wall). Collectively, our findings revealed that Atg8 maintains vacuolar membrane homeostasis in an autophagy-independent function by coordinating with other cellular factors.

The HOPS tethering complex is required to maintain signaling endosome identity and TORC1 activity

“…Ivy1 is an effector of Ypt7, binds PI3P, and can inhibit Fab1 ( Numrich et al, 2015 ; Malia et al, 2018 ; Lazar et al, 2002 ). It also dynamically relocalizes from puncta to vacuoles and vacuolar microdomains in response to nutrient starvation or cellular stress ( Numrich et al, 2015 ; Zweytick et al, 2014 ; Varlakhanova et al, 2018a ; Ishii et al, 2019 ). We showed before that a fraction of Ivy1 colocalizes with the EGO complex (EGOC) in endosomal dots, which are distinct from MVBs, as they lack Vps4 ( Chen et al, 2021 ; Hatakeyama et al, 2019 ).…”

Membrane recruitment of Atg8 by Hfl1 facilitates turnover of vacuolar membrane proteins in yeast cells approaching stationary phase

“…These data imply that a EVT-like process may exist in plants, although whether it involves any of the many plant Atg8 homologs remains to be determined. Lipidation-independent roles of Atg8 in the regulation of vacuoles have also been reported by several groups [58,59], although the significance of Hfl1-Atg8 interaction has only been documented in two yeast systems, Schizosaccharomyces pombe and Saccharomyces cerevisiae [50]. Interestingly, data from Liu et al and us both demonstrated that direct fusion of Atg8 to an unrelated vacuolar membrane protein was insufficient to circumvent Hfl1-Atg8 interaction.…”

Physicochemical properties of the vacuolar membrane and cellular factors determine formation of vacuolar invaginations

“…In addition, we reported previously that formation of vacuolar invaginations is a regulated process (Ishii et al, 2019). Atg8, a member of the ubiquitin-like family and one of the core elements involved in autophagy, suppresses vacuolar invaginations after heat stress (review in (Mizushima, 2020), (Ishii et al, 2019)). The functions of Atg8 could be divided into two categories, autophagy and autophagy-independent functions.…”

“…Since the Atg8's suppressing function of vacuolar invagination after heat stress does not need its lipidation, it was proposed to be one of autophagy-independent functions (Ishii et al, 2019).…”

“…Atg8’s autophagy-independent functions, including those in vesicular transport, resistance to oxidative stress, vacuolar fusion, and the formation of lipid bodies, have happened not to require the lipidation of Atg8 (Legesse-Miller et al, 2000; Maeda et al, 2017; Mikawa et al, 2010; Tamura et al, 2010). Since the Atg8’s suppressing function of vacuolar invagination after heat stress does not need its lipidation, it was proposed to be one of autophagy-independent functions (Ishii et al, 2019).…”