Protease trafficking in two primitive eukaryotes is mediated by a prodomain protein motif

Trypanosome protozoa, an early lineage of eukaryotic cells, have proteases homologous to mammalian lysosomal cathepsins, but the precursor proteins la ck mannose 6-phosphate. Utilizing green fluorescent protein as a reporter, we demonstrate that the carbohydrate-free prodomain of a trypanosome cathep sin L is necessary and sufficient for directing green fluorescent protein t o the lysosome/endosome compartment. A proper prodomain/catalytic domain pr ocessing site sequence is also required to free the mature protease for del ivery to the lysosome/endosome compartment. A nine-amino acid prodomain loo p motif, implicated in prodomain-receptor interactions in mammalian cells, is conserved in the protozoa, Site-directed mutagenesis now confirms the im portance of this loop to protease trafficking and suggests that a protein m otif targeting signal for lysosomal proteases arose early in eukaryotic cel l evolution.