AID-ing Signaling in Toxoplasma gondii

The cyclic GMP-dependent protein kinase (PKG) of apicomplexan parasites is essential for secretion of micronemes and host cell invasion and egress. Both kinase specificity and localization can determine which substrates are phosphorylated. The functions of plasma membrane and cytosolic PKG isoforms of Toxoplasma gondii were unknown because of difficulties precisely manipulating expression of essential genes. Brown et al. (K. M. Brown, S. Long, and L. D. Sibley, mBio 8:e00375-17, https://doi.org/10.1128/mBio.00375-17) adapted the auxin-inducible degron (AID) system for conditional expression of T. gondii proteins. AID, in combination with clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 gene editing, facilitated creation of a panel of PKG mutants to demonstrate that the membrane association via acylation of PKG is critical for its essential functions in tachyzoites. The cytosolic form of PKG is not sufficient for viability and is dispensable. These studies illuminate a critical role for targeting of kinase complexes for parasite viability. The AID system enables rapid, conditional regulation of protein expression that expands the molecular toolbox of T. gondii.