A Role for a 70-Kilodalton Heat Shock Protein in Lysosomal Degradation of Intracellular Proteins

A 73-kilodalton (kD) intracellular protein was found to bind to peptide regions that target intracellular proteins for lysosomal degradation in response to serum withdrawal. This protein cross-reacted with a monoclonal antibody raised to a member of the 70-kD heat shock protein (hsp70) family, and sequences of two internal peptides of the 73-kD protein confirm that it is a member of this family. In response to serum withdrawal, the intracellular concentration of the 73-kD protein increased severalfold. In the presence of adenosine 5′-triphosphate (ATP) and MgCl 2 , the 73-kD protein enhanced protein degradation in two different cell-free assays for lysosomal proteolysis.