Chloroplast heat shock protein cpHsc70-1 interacts with thylakoid membrane remodeling protein VIPP1 C-terminal tail and controls VIPP1 oligomer assembly

Oxygenic photosynthetic organisms depend on the thylakoid membranes (TMs) for light-driven energy conversion. Recent studies on TM homeostasis (thylakostasis) have highlighted the essential role of the TM remodeling protein vesicle-inducing protein in plastid 1 (VIPP1). As a member of the endosomal sorting complexes required for transport-III (ESCRT-III)/phage shock protein A (PspA)/VIPP1 superfamily, VIPP1 forms large ring- and filament-like homo-oligomeric structures that exhibit a membrane remodeling activity. The oligomerization status was proposed to be modulated by the intrinsically disordered C-terminal tail (Vc), whereas its functional role remained unclear. Notably, this Vc region is conserved not only in photosynthetic VIPP1 but also in the PspA proteins of extremophilic species, implicating its role in membrane stress responses. To investigate the role of the Vc region in VIPP1 assembly, we performed coimmunoprecipitation assays in Arabidopsis chloroplasts and identified chloroplast-localized HSP70 proteins (cpHsc70) as major interactors. Among the two isoforms, cpHsc70-1 was found to be specifically required for modulating VIPP1 oligomeric assembly and dynamics in response to heat stress. Genetic analyses revealed that cpHsc70-1 facilitates the disassembly of VIPP1 oligomers, similarly to Vps4 ATPase in ESCRT-III; loss of either the Vc region or cpHsc70-1-impaired VIPP1 disassembly, resulting in more static oligomeric structures. Furthermore, cpHsc70-1 exhibited a broader role in chloroplast proteostasis, as the cphsc70-1 mutant showed impaired accumulation of green fluorescent protein (GFP)-fusion proteins. Together, our findings uncover a crucial crosstalk between proteostasis and thylakostasis in chloroplasts, coordinated by cpHsc70-1 and VIPP1 in response to membrane stress.