Toxoplasma Cathepsin Protease B and Aspartyl Protease 1 Are Dispensable for Endolysosomal Protein Digestion

• Published in: mSphere 2020-02, Vol.5 (1)
• Main Authors: McDonald, Christian, Smith, David, Di Cristina, Manlio, Kannan, Geetha, Dou, Zhicheng, Carruthers, Vern B
• Format: Article
• Language: English
• Subjects: Aspartic Acid Proteases - metabolism; Aspartic endopeptidase; Cathepsin B - metabolism; Cathepsin D; Cathepsin L; Cell cycle; Chronic infection; Cysteine proteinase; Genomes; Hemoglobin; Host-Microbe Biology; Humans; Intracellular; Life Cycle Stages; Lysosomes - metabolism; Mammals; Parasites; Phylogenetics; Protein turnover; Proteins; Proteolysis; Proteolytic enzymes; Protozoan Proteins - metabolism; Toxoplasma; Toxoplasma - enzymology; Toxoplasma - growth & development; Toxoplasma gondii; Toxoplasmosis; Vacuoles - metabolism
• ISSN: 2379-5042; 2379-5042
• DOI: 10.1128/mSphere.00869-19

The lysosome-like vacuolar compartment (VAC) is a major site of proteolysis in the intracellular parasite Previous studies have shown that genetic ablation of a VAC-residing cysteine protease, cathepsin protease L (CPL), resulted in the accumulation of undigested protein in the VAC and loss of parasite viability during the chronic stage of infection. However, since the maturation of another VAC localizing protease, cathepsin protease B (CPB), is dependent on CPL, it remained unknown whether these defects result directly from ablation of CPL or indirectly from a lack of CPB maturation. Likewise, although a previously described cathepsin D-like aspartyl protease 1 (ASP1) could also play a role in proteolysis, its definitive residence and function in the endolysosomal system were not well defined. Here, we demonstrate that CPB is not necessary for protein turnover in the VAC and that CPB-deficient parasites have normal growth and viability in both the acute and chronic stages of infection. We also show that ASP1 depends on CPL for correct maturation, and it resides in the VAC, where, similar to CPB, it plays a dispensable role in protein digestion. Taken together with previous work, our findings suggest that CPL is the dominant protease in a hierarchy of proteolytic enzymes within the VAC. This unusual lack of redundancy for CPL in makes it a single exploitable target for disrupting chronic toxoplasmosis. Roughly one-third of the human population is chronically infected with the intracellular single-celled parasite , but little is known about how this organism persists inside people. Previous research suggested that a parasite proteolytic enzyme, termed cathepsin protease L, is important for persistence; however, it remained possible that other associated proteolytic enzymes could also be involved in the long-term survival of the parasite during infection. Here, we show that two proteolytic enzymes associated with cathepsin protease L play dispensable roles and are dependent on cathepsin L to reach maturity, which differs from the corresponding enzymes in humans. These findings establish a divergent hierarchy of proteases and help focus attention principally on cathepsin protease L as a potential target for interrupting chronic infection.