Cell biology: The TORC1 pathway to protein destruction.

Abstract

A study of the proteasome — a protein-degradation complex — reveals an evolutionarily conserved pathway that acts through the protein kinase TORC1 to adjust proteasome levels in response to cellular needs. See Letter p.184 The proteasome is the cell's protein degradation machinery. It is composed of a core particle and two regulatory particles. Chaperone proteins known as RACs mediate the assembly of the regulatory particles, and under stressful conditions in yeast, an additional chaperone called Adc17 helps to generate more proteasomes. A long-standing question has been how stress leads to increased Adc17 levels. Adrien Rousseau and Anne Bertolotti report that an evolutionarily conserved signalling pathway controls proteasome homeostasis in yeast and mammals. Specifically, they find that in an adaptive response to stress, the TORC1 protein in yeast (mTOR in mammals) is inhibited. Consequently, the downstream signalling molecule — the enzyme Mpk1 in yeast or Erk5 in mammals — ensures increased supplies of not just RACs, but also proteasome subunits.