In starving cells, ULK1 and ULK2 gather on incipient autophagosomes (yellow dots).

Cells take recycling to the extreme. If they run low on food, they digest and reuse some of their own cytoplasm. Hara et al. have discovered a key protein that helps get this process started.

This cellular self-eating is called autophagy. A membrane pouch known as an autophagosome forms inside the cell and swallows some cytoplasm. The autophagosome then hauls its contents to a lysosome for digestion. Researchers have teased out many of the molecular details of autophagy in yeast. But they know much less about how it works in mammals.

Hara et al. probed the function of two mouse kinases, ULK1 and ULK2. The proteins are homologous to a yeast protein that's vital for autophagy, but researchers weren't sure whether they performed the same job in mice. Hara et al. found that nonfunctional versions of ULK1 and ULK2 blocked autophagy and that the normal proteins clung to the membrane of autophagosome precursor compartments.

The team also captured one of ULK1 and ULK2's accomplices, FIP200. This protein gets around: it blocks tumor growth, hinders cell movement, helps control cell size, checks cell cycle progression, and prevents apoptosis. FIP200 bound ULK1 and ULK2 and ensured these proteins retained their kinase activity, the researchers found. Furthermore, mouse cells that lacked FIP200 demonstrated that this protein was also essential for autophagy.

How does a protein with so many responsibilities choose which one to perform? Hara et al. think that its choice of partner dictates its function. The team concludes that if FIP200 combines with ULK1 and ULK2, it helps instigate an early step of autophagy. But why this protein partnership whets a cell's appetite for itself remains unknown.

Hara, T., et al.
J. Cell Biol.