The molecular mechanism of autophagy.

Autophagy is a conserved trafficking pathway that is highly regulated by environmental conditions. During autophagy, portions of cytoplasm are sequestered into a double-membrane autophagosome and delivered to a degradative organelle, the vacuole in yeast and the lysosome in mammalian cells, for breakdown and recycling. Autophagy is induced under starvation conditions and in mammalian cells is also invoked in response to specific hormones. In yeast, under nutrient-rich conditions, a constitutive biosynthetic pathway, termed the cytoplasm to vacuole targeting (Cvt) pathway, utilizes most of the same molecular machinery and topologically similar vesicles for the delivery of the resident hydrolase aminopeptidase I to the vacuole. Both autophagy and the Cvt pathway have been extensively studied and comprehensively reviewed in the past few years. In this review, we focus on the yeast system, which has provided most of the insight into the molecular mechanism of autophagy and the Cvt pathway, and highlight the most recent additions to our current knowledge of both pathways.