Lysosomal degradation of microinjected proteins.

In this article we summarize evidence for a pathway by which cytosolic proteins can be selectively taken up and degraded within lysosomes. Serum deprivation of cells in culture activates this pathway, and only proteins that contain peptide sequences related to KFERQ (lysine, phenylalanine, glutamic acid, arginine, glutamine) are degraded at enhanced rates. Approximately 30% of intracellular proteins contain such peptide sequences, and we speculate about the physiological relevance of the selective degradation of these proteins in response to serum withdrawal. Several rat tissues also contain proteins with peptide sequences related to KFERQ, and the amount of these proteins is reduced in response to starvation. Finally, we present recent results suggesting that this selective uptake of cytosolic proteins by lysosomes is not through classical macroautophagic pathways. Instead, the selective uptake may be similar to other protein sorting pathways such as protein translocation through the endoplasmic reticulum or protein import into mitochondria.