Rim aperture of autophagic membranesbalances cargo inclusion with vesicle maturation

Abstract

Autophagy eliminates cytoplasmic material by engulfment in membranous vesicles targeted for lysosome degradation. Nonselective autophagy coordinates the sequestration of bulk cargo with the growth of the isolation membrane (IM) in a yet-unknown manner. Here we show the yeast IM expands while maintaining a rim sufficiently wide for sequestration of large cargo but tight enough to mature in due time. An obligate complex of Atg24/Snx4 with Atg20 or Snx41 assembles locally at the rim in a spatially-extended manner that specifically depends on autophagic PI(3)P. This assembly stabilizes the open rim to promote autophagic sequestration of large cargo in correlation with vesicle inflation. Moreover, constriction of the rim by the PI(3)P-dependent Atg2-Atg18 complex and clearance of PI(3)P by Ymr1 antagonize the rim opening to promote autophagic maturation and consumption of small cargo. Tight regulation of membrane rim aperture by PI(3)P thus couples the mechanism and physiology of nonselective autophagy.