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.