The regulation of autophagy in eukaryotic cells: do all roads pass through Atg1?

The induction of autophagy appears to be tightly controlled in all eukaryotic cells. This highly conserved, degradative process is induced by a variety of signals, including nutrient deprivation, and is generally thought to be incompatible with rapid cell growth. Recent work in the budding yeast, Saccharomyces cerevisiae, has suggested that the Atg1 protein kinase is at the center of this control. Atg1, and its associated proteins, appear to be directly targeted by multiple signaling pathways important for the control of both autophagy and cell growth. These pathways involve the small GTP-binding Ras proteins, the Tor protein kinases and the AMP-activated protein kinase, Snf1, respectively. A key question that remains is whether this regulatory paradigm has been evolutionarily conserved. In other words, is Atg1 the primary target of those signaling pathways responsible for coordinating growth with environmental influences in other eukaryotes? Here, we suggest that Atg1 is very likely to fulfill this role but that a truly definitive answer will require that we develop a better understanding of this protein kinase and its targets in all eukaryotes.