Cross-talk between PKA and PKG controls pH-dependent host cell egress of Toxoplasma gondii

Toxoplasma gondii encodes three Protein Kinase A catalytic (PKAc1-3) and one regulatory (PKAr) subunits to integrate cAMP-dependent signals. Here, we show that inactive PKAc1 is maintained at the parasite pellicle by interacting with dually acylated PKAr. Either a conditional knockdown of PKAr or the overexpression of PKAc1 results in a block in parasite division. In contrast, conditional expression of a dominant negative PKAr isoform unable to bind cAMP, triggers premature egress of parasites from infected cells. This untimely egress critically depends on parasite density and host cell acidification. A comparative phosphoproteome analysis reveals that PKA genetic inhibition significantly changed the phosphorylation profile of a putative cGMP-phosphodiesterase, PDE2. Consistently, the phenotype of PKA genetic inhibition is alleviated by chemical inhibition of the cGMP-dependent protein kinase G (PKG). A phosphodiesterase inhibitor is able to circumvent egress repression by PKA or pH neutralisation, indicating that environmental acidification and PKA signalling act as balancing regulators of cGMP degradation to control PKG-mediated egress. Collectively, these results reveal a cross-talk between PKA and PKG pathways to govern egress in T. gondii.