The Five Stages of Autophagy and the Lysosome

Zoe "Doctor, as you suggested, last week I searched online for autophagy. Yoshinori Ohsumi of Japan won the 2016 Nobel Prize in Physiology or Medicine for it. The citation says he discovered the genes related to autophagy in yeast. But isn't yeast just the microbe used to brew beer and bake bread?"

Dr. Lee "Yes. Yeast is widely used in beer-making and bread-making, but although small, it is a eukaryote and therefore widely used in genetic research. A eukaryotic cell has a nuclear membrane that encloses its DNA. Metabolically, yeast also has mitochondria, which allow significant energy production. Remarkably, even when 47 percent of yeast genes are replaced with human equivalents, the yeast still survives."

Emily "Why are autophagy-related genes needed in the first place?"

Dr. Lee "Genes exist to produce proteins. To understand this, it helps to first see how autophagy unfolds (Mizushima & Komatsu, 2011; Yang & Klionsky, 2010; Glick et al., 2010). As we discussed last time, autophagy is the process of breaking down waste inside the cell. That waste includes damaged mitochondria, aggregated proteins, aggregated lipids, aggregated carbohydrates, and damaged genetic material. When does this internal cleanup actually take place?"

Zoe "You said last time, Doctor — that autophagy starts when we go hungry, that is, when we fast."

Dr. Lee "Yes. When we don't eat, no nutrients reach the cells from outside. But the cells still need energy to survive, so they begin using nutrients stored inside themselves. The first thing they reach for is the waste that's been sitting there."