Control of autophagosome size and number by Atg7.

The induction of bulk autophagy by nitrogen starvation in baker's yeast (S. cerevisiae) involves the upregulation of many autophagy related proteins, including Atg7. One way to investigate the importance of this upregulation is to measure the size and number of autophagosomes formed when insufficient amounts of that protein are available. Atg8 is known to affect autophagosome size, consistent with its role in phagophore expansion. Atg7 is upstream of Atg8, and might therefore be expected to affect only autophagosome size. We used electron microscopy to measure the size and number of autophagosomes formed with limiting amounts of Atg7 and found them to be both smaller and fewer than normal. This suggests that Atg7 may have an Atg8-independent role in autophagosome initiation in addition to its Atg8-dependent role in autophagosome expansion. We also present an improved simulation for estimating original autophagic body number based on the number of cross-sections observed in ultrathin sections.