Proteomic profile of Toxoplasma gondii stress granules by high resolution mass spectrometry.

Ribonucleoprotein granules are bio-condensates which form a diverse group of dynamic membrane-less organelles implicated in several cellular functions including stress response and cellular survival. In Toxoplasma gondii, a type of bio-condensates referred to as stress granules (SGs) are formed prior to the parasites' egress from the host cell and are implicated in the survival and invasion competency of extracellular tachyzoites. We used paraformaldehyde to fix and cross-link SG proteins to allow purification by centrifugation and analysis by mass spectrometry. We profiled protein components of SGs at 10 min and 30 min post-egress when parasite's invasion ability is significantly diminished. Thirty-three proteins were identified from 10-min SGs, and additional 43 proteins were identified from 30-min SGs. Notably, common SG components such as proteins with intrinsically disordered domains were not identified. Gene ontology analysis of both 10-min and 30-min SGs shows that overall molecular functions of SGs' proteins are ATP-binding, GTP-binding and GTPase activity. Discernable differences between 10-min and 30-min SGs are in the proportions of translation and microtubule related proteins. 10-min SGs have a higher proportion of microtubule-related proteins and a lower proportion of ribosome related proteins, while a reverse correlation was identified for those of 30-min. It remains to be investigated whether this reverse correlation contributes to the ability of extracellular tachyzoites to reinvade host cells.