Peptide sequences that target proteins to lysosomes for enhanced degradation during serum withdrawal

Ribonuclease A (RNase A) microinjected into human diploid fibroblasts is degraded with a half-life of 80-100 h in the presence of serum, but its half-life declines to 40 h when cells are deprived of serum. This increased degradation in response to serum withdrawal results from an increased rate of uptake of microinjected RNase A from the cytosol into lysosomes. The cells' ability to recognize RNase A for this enhanced degradation is based on some feature of amino acids 1-20. Covalent linkage of S-peptide to other proteins results in enhanced lysosomal degradation of the conjugate in response to serum withdrawal. The authors have further defined the essential region of RNase S-peptide to be within residues 7-11, KFERQ. Coinjection of radiolabeled RNase A and excess unlabeled pentapeptide specifically blocks the enhanced degradation of RNase A. Affinity-purified polyclonal IgGs raised against KFERQ immunoprecipitate 25-30% of radiolabeled cytosolic proteins from human fibroblasts. Pulse-chase experiments indicate that these proteins are preferentially degraded upon serum withdrawal while degradation of nonimmuno-precipitated proteins is unaffected. These results suggest that peptide sequences similar to KFERQ are contained within cellular proteins and that such sequences target proteins to lysosomes for enhanced degradation during serum withdrawal.