Endocytosis of liposomes containing lysosomal proteins increases intracellular protein degradation in growing L-132 cells.

We have used a new approach to test the possible participation of lysosomes in the degradation of long-lived proteins. Rat liver lysosomal proteins were introduced, via multilamellar liposomes, into L-132 cells. Viability and protein synthesis were not impaired by this treatment. The liposomal content was released into the lysosomes of the cultured cells, as revealed by ferritin uptake and electron microscopy. Degradation rates of long-lived proteins increased with the uptake of lysosomal proteases. However, the increased protein degradation of chloroquine and leupeptin, in contrast to the inhibition by these reagents of the increased protein degradation of cells 'starved' of serum (step-down conditions). This approach opens a new way of investigating the degradation of intracellular proteins in cultured cells.