Cell Invasion

As an obligate intracellular pathogen, Toxoplasma critically relies on cell invasion as a major survival strategy to avoid host antibody defense and phagocytic clearance. Cell invasion also initiates the parasite lytic cycle that ultimately destroys the infected cell, causing direct tissue pathology and indirect inflammatory damage. Our collaborators and we have shown that Toxoplasma uses a battery of adhesive protein complexes to recognize and bind host cells prior to invasion. Many of these adhesins reside in specialized secretory organelles called micronemes (Greek: small threads), which are discharged when the parasite has identified a suitable site for cell invasion. We have shown that parasites conditionally deficient in one particular complex (MIC2-M2AP) are invasion incompetent, partially defective in gliding motility, and fail to affect mice even at high doses (1). Our team has revealed how adhesive complexes such as MIC2-M2AP are assembled, proteolytically processed and shuttled to invasion organelles (2-5). We have also identified proteases that activate invasion proteins along with a protein called MIC5 that structurally mimics the autoregulatory propeptide of a serine protease called SUB1 (4,5). To our knowledge this the the first example of such mimicry for regulation of a subtilisin protease. Our most recent work involves determining the function of poorly characterized invasion-related proteins that are conserved amongst Toxoplasma's kin including the malaria parasite. We showed one of these conserved proteins, SPATR, contributes to Toxplasma cell invasion, likely by facilitating parasite attachment to a target cell (6). SPATR and other proteins under investigation are expressed in multiple life stages of T. gondii and malaria parasites, making them attractive vaccine candidates. Understanding the functions of such conserved invasion proteins has the potential to widely impact the development of vaccines or novel therapeutics.

1. Huynh, M.-H., Carruthers, V.B. (2006) Toxoplasma MIC2 is a major determinant of invasion and virulence. PLoS Pathog Aug 18;2(8).

2. Gaji, R., Flammer, H.P., and Carruthers, V.B. (2011) Forward targeting of Toxoplasma gondii proproteins to the micronemes involves conserved aliphatic amino acids. Traffic. 12;840-853.

3. Saouros*, S., Dou*, Z., Henry, M., Simpson, P., Carruthers, V.B., and Matthews, S. (2012) Microneme protein 5 regulates Toxoplasma subtilisin 1 activity by mimicking a subtilisin prodomain. J. Biol. Chem. 287:36029-36040. *equal contributors

4. Parussini, F., Coppens, I., Shah, P., Diamond, S.L., and Carruthers, V.B. (2010) Cathepsin L occupies a vacuolar compartment and is a protein maturase within the endo/exosomal system of Toxoplasma gondii. 76;1340-1357.

5. Huynh, M.-Y., Liu, B., Henry, M., Liew, L., Matthews, S.J., Carruthers, V.B. (2015) Structural basis of Toxoplasma gondii MIC2-associated protein interaction with MIC2. J. Biol. Chem. 290(3):1432-41.

6. Huynh, M.-Y., Boulanger, M.J., Carruthers, V.B. (2014) A conserved apicomplexan microneme protein contributes to T. gondii invasion and virulence. Inf. Immun. 82:4358-4368.