The rate of proteolysis is an important determinant of the intracellular protein 
content. Part of the degradation of intracellular proteins occurs in the 
lysosomes and is mediated by macroautophagy. In liver, macroautophagy is very 
active and almost completely accounts for starvation-induced proteolysis. 
Factors inhibiting this process include amino acids, cell swelling and insulin. 
In the mechanisms controlling macroautophagy, protein phosphorylation plays an 
important role. Activation of a signal transduction pathway, ultimately leading 
to phosphorylation of ribosomal protein S6, accompanies inhibition of 
macroautophagy. Components of this pathway may include a heterotrimeric 
Gi3-protein, phosphatidylinositol 3-kinase and p70S6 kinase. Recent evidence 
indicates that lysosomal protein degradation can be selective and occurs via 
ubiquitin-dependent and -independent pathways.