ABSTRACT
The intracellular protozoan
Toxoplasma gondii
dramatically reprograms the transcriptome of host cells it infects, including substantially up-regulating the host oncogene c
-myc
. By applying a flow cytometry-based selection to infected mouse cells expressing green fluorescent protein fused to c-Myc (c-Myc–GFP), we isolated mutant tachyzoites defective in this host c-Myc up-regulation. Whole-genome sequencing of three such mutants led to the identification of
MYR1
(
My
c
r
egulation
1
;
TGGT1_254470
) as essential for c-Myc induction. MYR1 is a secreted protein that requires TgASP5 to be cleaved into two stable portions, both of which are ultimately found within the parasitophorous vacuole and at the parasitophorous vacuole membrane. Deletion of
MYR1
revealed that in addition to its requirement for c-Myc up-regulation, the MYR1 protein is needed for the ability of
Toxoplasma
tachyzoites to modulate several other important host pathways, including those mediated by the dense granule effectors GRA16 and GRA24. This result, combined with its location at the parasitophorous vacuole membrane, suggested that MYR1 might be a component of the machinery that translocates
Toxoplasma
effectors from the parasitophorous vacuole into the host cytosol. Support for this possibility was obtained by showing that transit of GRA24 to the host nucleus is indeed MYR1-dependent. As predicted by this pleiotropic phenotype, parasites deficient in
MYR1
were found to be severely attenuated in a mouse model of infection. We conclude, therefore, that MYR1 is a novel protein that plays a critical role in how
Toxoplasma
delivers effector proteins to the infected host cell and that this is crucial to virulence.
IMPORTANCE
Toxoplasma gondii
is an important human pathogen and a model for the study of intracellular parasitism. Infection of the host cell with
Toxoplasma
tachyzoites involves the introduction of protein effectors, including many that are initially secreted into the parasitophorous vacuole but must ultimately translocate to the host cell cytosol to function. The work reported here identified a novel protein that is required for this translocation. These results give new insight into a very unusual cell biology process as well as providing a potential handle on a pathway that is necessary for virulence and, therefore, a new potential target for chemotherapy.
A revised mechanism for how Plasmodium falciparum recruits and exports proteins into its erythrocytic host cell.
