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Your cells are constantly taking themselves apart. Worn-out proteins and even whole organelles are wrapped up, delivered to a digestive compartment and broken into reusable pieces. That process, autophagy, had been seen since the 1960s, but almost nothing was known about how it was controlled. Yoshinori Ohsumi changed that using baker's yeast. This post covers who he is, the two key experiments, a worked example on reading his genetic screen, and what students can learn from his approach.
Who is Yoshinori Ohsumi?
The Nobel Prize facts page for Ohsumi records that he was born on 9 February 1945 in Fukuoka, Japan, and received his doctorate from the University of Tokyo in 1974. After a few years at Rockefeller University in New York he returned to Tokyo, moved to the National Institute for Basic Biology in Okazaki in 1996, and was at the Tokyo Institute of Technology when he received the 2016 Nobel Prize in Physiology or Medicine "for his discoveries of mechanisms for autophagy". He was the sole laureate.
What was known about autophagy before Ohsumi?
The Nobel Assembly's 2016 press release explains that autophagy means "self eating". In the 1960s, researchers saw cells enclose their own contents in membranes and send them to the lysosome, the compartment full of digestive enzymes. Christian de Duve, who discovered the lysosome, coined the word, and the new vesicles were named autophagosomes. Our post on de Duve and the discovery of lysosomes tells that part of the story.
What was missing was the machinery. The press release says difficulties in studying the process meant little was known until Ohsumi's experiments in the early 1990s.
How did Ohsumi make autophagy visible in yeast?
When Ohsumi started his own lab in 1988, the press release says, he focused on protein breakdown in the yeast vacuole, which corresponds to the lysosome in human cells. Yeast cells are small, and he was not even sure autophagy happened in them.
His idea was simple: if the vacuole could not digest what it received, the delivered material should pile up where it could be seen. In the 1992 paper by Takeshige, Ohsumi and colleagues in the Journal of Cell Biology, the team moved yeast lacking proteinases A and B and carboxypeptidase Y from a nutrient medium to one lacking nutrients.
After 1 hour, a few spherical bodies appeared in the vacuoles, moving by Brownian motion.
After 3 hours, they filled the vacuoles almost completely.
Electron microscopy showed each body was surrounded by a membrane thinner than any other membrane in the cell and contained ordinary cytoplasm, including ribosomes and even mitochondria.
The bodies were 400 to 900 nm across. The team named them "autophagic bodies".
The paper calls this "the first report that nutrient-deficient conditions induce extensive autophagic degradation of cytosolic components in the vacuoles of yeast cells."
How did he find the genes?
Now he had a test that could be seen in a light microscope. In 1993, Tsukada and Ohsumi in FEBS Letters80398-E) mutagenised the proteinase-deficient yeast and looked for cells that failed to accumulate autophagic bodies when starved. The first mutant, named apg1, could not break down protein under nitrogen starvation and lost viability faster than normal cells. Using that faster death as a screen, they picked 75 more mutants. All 76 fell into 15 complementation groups, which means at least 15 genes are needed for autophagy in yeast. The Nobel press release adds that similar machinery was soon found to operate in human cells.
Worked example: reading a complementation test
A complementation test asks whether two recessive mutants are broken in the same gene. Cross them. If the hybrid is normal, each parent supplies a working copy of the gene the other lacks, so the mutations are in different genes. If the hybrid is still mutant, they are in the same gene.
Sample results (made-up numbers for practice, not Ohsumi's data). Five autophagy-defective mutants, M1 to M5, are crossed in pairs:
M1 x M2: mutant
M1 x M3: normal
M2 x M3: normal
M3 x M4: mutant
M1 x M5: normal, M3 x M5: normal, M4 x M5: normal
Group them: M1 and M2 share a gene; M3 and M4 share another; M5 complements everything tested, so it is a third gene. Five mutants, three complementation groups, so at least three genes. Ohsumi's screen did the same thing with 76 mutants and found 15 groups.
A size check. Autophagic bodies 400 and 900 nm across differ in diameter by a factor of 2.25, but in volume by 2.25^3 = about 11.4. Volume scales with the cube of diameter, which is easy to forget.
Common mistakes students make here
Confusing the vacuole and the lysosome. In yeast the vacuole does the lysosome's digestive job; they are not the same structure in every cell.
Thinking one mutant means one gene. Many mutants can hit the same gene. Only complementation tells you how many genes there are.
Calling autophagy simple destruction. The press release stresses recycling: the breakdown products supply nutrients and building blocks for renewal.
If organelle functions keep running together, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can review the cell one part at a time.
What can a student take from how Ohsumi worked?
Pick a system you can actually observe. Yeast is small, but blocking vacuolar digestion made the process visible.
Turn an observation into a screen. Once autophagic bodies could be seen, every mutant could be tested.
Use a model organism. The press release notes that yeast cells are easy to study and especially useful for finding genes in complex pathways.
Practice routine: for one NCERT cell organelle a day, write its function, one experiment that could test it, and what you would expect if that organelle failed.
Next step: draw the path from autophagosome to vacuole, then add where Ohsumi's proteinase-deficient mutation blocks it.
