Tamura et al. describe how phosphorylation modulates the phospholipid-binding capacity of a protein that regulates autophagy and vacuole morphology.
By binding to the phospholipid PI(3)P, Atg18 promotes assembly of the phagophore, a double-membraned structure that engulfs cytoplasmic contents ahead of their degradation during autophagy. By binding to PI(3,5)P2, on the other hand, Atg18 fragments the vacuole (the yeast equivalent of the lysosome) in response to a variety of environmental stresses. How these two functions are regulated is unclear.
Tamura et al. discovered that the phospholipid-binding domain of Atg18 is partially phosphorylated in the methylotrophic yeast Pichia pastoris and that this modification reduced the protein’s ability to bind PI(3,5)P2. Atg18 was dephosphorylated in response to conditions—including hyperosmotic stress—that stimulate vacuole fission, prompting the protein to bind the vacuole membrane and promote the organelle’s fragmentation. In contrast, conditions that induce vacuole fusion—such as hypo-osmotic stress—stimulated Atg18’s phosphorylation and dissociation from vacuole membranes.
Micropexophagy is a specialized form of autophagy that targets peroxisomes for degradation. The membrane that engulfs the peroxisomes is provided by vacuole fission and by the formation of a phagophore-like structure called the MIPA. Tamura et al. found that dephosphorylated Atg18 promoted the vacuole fission required for micropexophagy. However, Pichia cells also required a non-vacuole–associated pool of phosphorylated Atg18 to generate the MIPA, suggesting that cells coordinate the two membrane sources by regulating Atg18 phosphorylation levels. Senior author Yasuyoshi Sakai now wants to identify the kinase and phosphatase responsible for Atg18 regulation.
Text by Ben Short