Figure 1.

Autophagy and the Cvt pathway in yeast. Morphological steps are schematically shown in autophagy and the Cvt pathway. Both pathways require the sequestration of cargo components within distinct double-membrane vesicles, whose formation is thought to occur at the PAS. Autophagy is induced by inactivation of Tor kinase under starvation conditions such as nitrogen depletion. Organelles and bulk cytoplasm are sequestered into a double-membrane autophagosome, which is 300-900 nm in diameter. The Cvt pathway is a biosynthetic process that delivers resident hydrolases including Ape1 to the vacuole in vegetative conditions. The Cvt vesicle (140-160 nm in diameter) enwraps specific cargos such as the prApe1 oligomer (Ape1 complex). The Ape1 complex is also sequestered specifically by autophagosomes under starvation conditions. Once completed, the double-membrane vesicles target to and fuse with the vacuole, and the single-membrane vesicles (autophagic or Cvt body) are released into the lumen. Subsequently, these vesicles are degraded, allowing maturation of prApe1 into the mature form (mApe1) by removal of its propeptide, and the degradation of the cytoplasm. Fluorescent images show that the cargo protein CFP-Ape1 localizes at the perivacuolar PAS (white arrowhead) marked by YFP-Atg8; the vacuole is labeled with FM 4-64 in the merged image. DIC, differential interference contrast