Polypeptide import and degradation by isolated lysosomes

We reported previously that lysosomes derived from human diploid fibroblasts import and degrade polypeptides and that these processes are stimulated by ATP and by the heat shock cognate protein of 73 kDa (hsc73). We now report several new aspects of this in vitro proteolytic pathway, (a) Among four polypeptides tested, this pathway appears to be selective for those containing KFERQ-like peptide motifs. (b) Substrate proteins specifically bind to a protein-containing site on lysosomal membranes, (c) Lysosomes derived from serum-deprived cells are twice as active as those from serum-supplemented cells, (d) A portion of intracellular hsc73 is associated with lysosomes, and the amount of lysosomal hsc73 increases in response to serum withdrawal. Additional characterization of this proteolytic pathway not reported previously shows that intact lysosomes are required, the import process is saturable with an apparent K_m of 5 μM for RNase S-peptide, and reducing agents activate this lysosomal import and degradation pathway.