Identification of a mitochondrial targeting sequence in cathepsin D and its localization in mitochondria

Cathepsin D (CTSD) is a major lysosomal protease harboring an N-terminal signal peptide (amino acids 1-20) to enable vesicular transport from endoplasmic reticulum to lysosomes. Here, we report the possibility of a mitochondrial targeting sequence and mitochondrial localization of CTSD in cells. Live-cell imaging analysis with C-terminal enhanced green fluorescent protein-tagged CTSD (EGFP-CTSD) indicated that CTSD localizes to mitochondria. CTSD amino acids 21-35 are responsible for its mitochondrial localization, which exhibit typical features of mitochondrial targeting sequences, and are evolutionarily conserved. A proteinase K protection assay and sucrose gradient analysis showed that a small population of endogenous CTSD molecules exists in mitochondria. These results suggest that CTSD is a dual-targeted protein that may localize in both lysosomes and mitochondria.