Balanced activities of Atg2 and Atg24 regulate opening of the autophagic isolation membrane

Autophagy targets portions of the cytosol to lysosomal degradation via sequestration into a de novo built double-membrane vesicle termed autophagosome. Here we study the biogenesis of non-selective autophagosomes in yeast, employing a novel combination of visual

characterization of autophagic structures under fluorescence microscopy with a partial loss-of-function approach for timely and quantitative reduction of core autophagic machinery protein levels. We show that autophagosomes expand in the shape of a round amphora with a narrow opening, which is widened by the partial loss of Atg2 – in a manner requiring the in situ activity of Atg24. Accordingly, gain of Atg24 recruitment increases- while loss of Atg24 restricts- the opening of the amphora – with the latter correlates with flattening of the vesicle and exclusion of large cargo.

Our work thus revises the traditional model of promiscuous engulfment of the cytosol by an elongating cup of constant width, into a model of an expanding round membrane with a size-excluding opening, whose topology is highly regulated in a physiologically-relevant manner by distinct molecules.