Syntaxin 17 recruitment to mature autophagosomes is temporally regulated by PI4P accumulation

Electrostatic maturation of the autophagosome

In macroautophagy, lysosomes fuse with closed autophagosomes but not with unclosed ones. This is achieved, at least in part, by the temporally regulated recruitment of the autophagosomal SNARE STX17 (syntaxin 17) to only mature autophagosomes. However, the molecular mechanism by which STX17 recognizes autophagosomal maturation remains unknown. Our recent study revealed that STX17 recruitment is regulated by the electrostatic interaction between the positively charged C-terminal region of STX17 and the autophagosomal membrane, which becomes negatively charged during maturation due to the accumulation of phosphatidylinositol-4-phosphate (PtdIns4P). Here, we propose an electrostatic maturation model of the autophagosome.

ORP9-PH domain-based fluorescent reporters for visualizing phosphatidylinositol 4-phosphate dynamics in living cells

Fluorescent reporters that visualize phosphatidylinositol 4-phosphate (PI4P) in living cells are indispensable to elucidate the roles of this fundamental lipid in cell physiology. However, currently available PI4P reporters have limitations, such as Golgi-biased localization and low detection sensitivity. Here, we present a series of fluorescent PI4P reporters based on the pleckstrin homology (PH) domain of oxysterol-binding protein-related protein 9 (ORP9). We show that the green fluorescent protein AcGFP1-tagged ORP9-PH domain can be used as a fluorescent PI4P reporter to detect cellular PI4P across its wide distribution at multiple cellular locations, including the plasma membrane (PM), Golgi, endosomes, and lysosomes with high specificity and contrast. We also developed blue, red, and near-infrared fluorescent PI4P reporters suitable for multicolor fluorescence imaging experiments. Finally, we demonstrate the utility of the ORP9-PH domain-based reporter to visualize dynamic changes in the PI4P distribution and level in living cells upon synthetic ER-PM membrane contact manipulation and GPCR stimulation. This work offers a new set of genetically encoded fluorescent PI4P reporters that are practically useful for the study of PI4P biology.

A matter of timing

A change in the electric charge of autophagosome membranes controls the recruitment of SNARE proteins to ensure that membrane fusion occurs at the right time during autophagy.