Molecular mechanisms of ESCRT-mediated autophagosome maturation in plants

Diverse environmental stress factors affect the functionality of proteins and membrane compartments within cells causing potentially irremediable damage to the cell. A major process to eliminate nonfunctional molecular aggregates or damaged organelles under stress conditions is macroautophagy/autophagy, thus making its regulation critical for cellular adaptation and survival. The formation of autophagosomes is coordinated by a wide range of cellular factors and culminates in the closure of the cup-shaped double membrane or phagophore. The endosomal sorting complex required for transport (ESCRT) machinery has been proposed to mediate the sealing of the autophagic membranes. However, the molecular basis for ESCRT recruitment to phagophores under stress conditions are not yet fully understood. We recently described the role of ALIX (ALG-2 interacting protein-X) and its interactor CALB1 (Ca²⁺-dependent Lipid Binding protein 1) in autophagosome maturation during salt stress in Arabidopsis. Our study shows that CALB1 is important for phagophore closure and thus to the subsequent delivery to the vacuole. CALB1 localizes on salt-induced phagophores together with ALIX. CALB1 stimulates the phase separation of ALIX, which can facilitate the further ESCRT recruitment to phagophore membranes.