A polyphosphate-lon protease complex in the adaptation of Escherichia coli to amino acid starvation.

Cells must balance energy-efficient growth with the ability to adapt rapidly to sudden changes in their environment. For example, in an environment rich in amino acids, cells do not expend energy for making amino acid biosynthetic enzymes. However, if the environment becomes depleted of amino acids (nutritional downshift), cells will be exposed to a lack of both the amino acid biosynthetic enzymes and the amino acids required to make these enzymes. To solve this dilemma, cells must use their own proteins as sources of amino acids in response to the nutritional downshift. Once amino acid biosynthetic enzymes start to accumulate, the cell is able to produce its own amino acids, and a new growth phase begins. In Escherichia coli, amino acid starvation leads to the accumulation of an unusual molecule, polyphosphate (polyP), a linear polymer of many hundreds of orthophosphate residues. Protein degradation in this bacterium appears to be triggered by the accumulation of polyP. PolyP forms a complex with the ATP-dependent Lon protease. The formation of a complex then enables Lon to degrade free ribosomal proteins. Certain very abundant ribosomal proteins can be the sacrificial substrates targeted for degradation at the onset of the downshift. Here I propose to call the polyP-Lon complex the "stringent protease," and I discuss new insights of protein degradation control in bacteria.