Bridging Molecules: Exploring the Structural and Biochemical Interface between the Glideosome-Associated Connector and F-actin

Toxoplasma gondii relies on gliding motility for crucial processes such as invasion, egress, and traversing biological barriers. This motility is orchestrated by the glideosome, a molecular machine unique to apicomplexans. Gliding motility involves conoid extrusion and microneme secretion and is powered by the actomyosin system, including myosin H, myosin A, and filamentous actin produced by formin1 (FRM1). The glideosome-associated connector (GAC) plays a pivotal role in bridging the cytosolic tail of MIC2 to F-actin. GAC is a large protein conserved across Apicomplexa, composed of numerous armadillo repeats and a PH domain that binds to phosphatidic acid. Positioned initially at preconoidal rings, GAC translocates to the basal pole via the flux of F-actin during motility1.

The coordinated interactions of GAC with F-actin, adhesins, and membranes are essential for transmitting the power generated by motors and propelling the parasite forward. Determination of GAC structure by X-ray crystallography highlights a model composed of 53 armadillo repeats forming a large arch that folds back onto itself, making intramolecular interactions between N- and C-terminal ends2.In solution, GAC exhibits dynamic properties, adopting various conformations from closed to open and extended. Insights into interaction of GAC with F-actin were obtained using hydrogen/deuterium exchange coupled with mass spectrometry (HDX-MS). Results identified several F-actin binding sites that localise at the intramolecular contact site of GAC, predicting that a conformational change facilitating F-actin binding is required. Several mutants have been designed and are poised for testing in the parasite to unravel the contribution of critical GAC residues to parasite fitness and specific gliding functions.

1Tosetti, N. et al, Elife. 2019 Feb 12;8:e42669.

2Kumar, A. et al, Elife, 2023 Apr 4:12:e86049