Phosphoproteome of Toxoplasma gondii Infected Host Cells Reveals Specific Cellular Processes Predominating in Different Phases of Infection.

The invasion of Toxoplasma gondii tachyzoites into the host cell results in extensive host cell signaling activation/deactivation that is usually regulated by the phosphorylation/dephosphorylation. To elucidate how T. gondii regulates host cell signal transduction, the comparative phosphoproteome of stable isotope labeling with amino acids in cell culture-labeled human foreskin fibroblast cells was analyzed. The cells were grouped (Light [L], Medium [M], and Heavy [H] groups) based on the labeling isotope weight and were infected with T. gondii for different lengths of time (L: 0 hour; M: 2 hours; and H: 6 hours). A total of 892 phosphoproteins were identified with 1,872 phosphopeptides and 1,619 phosphorylation sites. The M versus L comparison revealed 694 significantly regulated phosphopeptides (436 upregulated and 258 downregulated). The H versus L comparison revealed 592 significantly regulated phosphopeptides (146 upregulated and 446 downregulated). The H versus M comparison revealed 794 significantly regulated phosphopeptides (149 upregulated and 645 downregulated). At 2 and 6 hours post-T. gondii infection, the most predominant host cell reactions were cell cycle regulation and cytoskeletal reorganization, which might be required for the efficient invasion and multiplication of T. gondii. Similar biological process profiles but different molecular function categories of host cells infected with T. gondii for 2 and 6 hours, which suggested that the host cell processes were not affected significantly by T. gondii infection but emphasized some differences in specific cellular processes at this two time points. Western blotting verification of some significantly regulated phosphoprotein phosphorylation sites was consistent with the mass spectra data. This study provided new insights into and further understanding of pathogen-host interactions from the host cell perspective.