Methionine is a regulator of starvation-induced proteolysis in Tetrahymena.

The ciliate Tetrahymena thermophila responds to starvation by drastically increasing the rate of proteolysis. The response was reversed by resuspending the cells in a defined growth medium. Among the components of this medium only amino acids were active in inhibiting proteolysis. One amino acid, methionine, accounted for at least 75% of the effect of the complete medium, strongly indicating that in Tetrahymena methionine is the main regulator of step-down proteolysis, a process generally connected with autophagy in eukaryotic cells. The fact that one amino acid has such a drastic effect should make the system well suited for further investigations of the regulation of this process.