Protein degradation: Maintaining supplies

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Proteasomal protein degradation during sudden amino-acid deprivation ensures the translation of new proteins.

A new role for the proteasome in protecting mammalian cells against sudden amino-acid deprivation has been discovered by Vabulas and Hartl. A decreased external amino-acid supply is known to result in an upregulation of the autophagosomal–lysosomal pathway, which provides free amino acids for protein synthesis by degrading cytoplasmic proteins and organelles. However, this adaptation takes time. In Science, Vabulas and Hartl now show that, in the interim period, efficient protein translation is maintained by proteasomal protein degradation.

The authors first established a system in which they could measure the immediate effects of proteasome inhibition in HeLa cells. They selected a rapidly degraded ubiquitin-tagged protein as a reporter of proteasome activity, and showed that the simultaneous addition of a proteasome inhibitor and an inhibitor of protein synthesis (which is known to work quickly) stabilized reporter levels. So, because the level of the reporter did not decrease, proteasome inhibition was immediate.

The effect of this immediate proteasome inhibition on protein translation was subsequently analysed under conditions of acute amino-acid restriction. Proteasome inhibition was shown to impair translation only when cells were incubated in a medium that lacked at least one essential amino acid. This indicates that the proteasome has an essential role in allowing translation to proceed when amino-acid supplies are suddenly limited. Indeed, inhibiting the proteasome during prolonged amino-acid restriction did not affect translation. Furthermore, during acute amino-acid restriction, translation was unaffected by inhibiting the autophagosomal–lysosomal pathway.

Amino-acid analysis showed directly that the proteasome provides free amino acids for protein synthesis. After the inhibition of protein synthesis in a Leu-deficient medium, intracellular Leu could only be detected if the proteasome was not inhibited.

So, which proteins does the proteasome degrade to maintain the amino-acid supply? It has previously been reported that 30% of proteins are degraded by the proteasome during and immediately after translation. However, the data of Vabulas and Hartl contradict this finding. If this percentage of nascent and newly synthesized proteins is, in fact, degraded, proteasome inhibition would result in their accumulation, and this effect was not seen.

In fact, the authors showed that translating polypeptides are protected against proteasomal degradation, even when they are unable to fold. Proteins that were made in the presence of a Pro analogue, which prevents correct folding, were not degraded immediately, but were instead degraded through a relatively slow post-translational process. It therefore seems that it is the proteasomal degradation of pre-existing proteins that maintains amino-acid supplies under conditions of acute amino-acid restriction.

Rachel Smallridge

References

Vabulas, R. M. & Hartl, F. U. Protein synthesis upon acute nutrient restriction relies on proteasome function. Science 310, 1960–1963 (2005) | Article | PubMed |

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