Authors

F Latorre-Barragan²; J Whitelaw²; S Gras²; G Pall²; J Leung¹; G Ward¹; M Meissner²;  ¹ University of Vermont, United States;  ² WTCMP University of Glasgow

Discussion

In Toxoplasma gondii motility and invasion is thought to rely on the actomyosin- system. The main motor complex, consisting of myosin A, its light chain (MLC1) and gliding associated proteins (GAPs), is anchored to the inner membrane complex (IMC), and is thought to produce the force on short actin filaments (ACT1) for gliding and invasion. This mechanical force is transmitted to transmembrane proteins, which are translocated in a directional manner to the basal end of the parasite, resulting in forward motion on the substrate. Interestingly, using reverse genetics, it has been demonstrated that MyoA, MLC1 or ACT1 are not essential for gliding and invasion, necessitating a reassessment of the individual functions of the key molecules of the complex.

One plausible explanation of this surprising finding is the presence of compensatory myosin motors that can take over the role of MyoA. Due to its structural similarity with MyoA, we studied the possibility of myosin C (MyoC) taking over MyoA function. We generated three different complementation constructs, and compared their expression in the myoA KO. Our results suggest that IMC-localised MyoC can partially complement for MyoA in terms of invasion and gliding rates, but not average speed or gliding distance, indicating that MyoC cannot compensate the motor function of MyoA per se. Moreover, depletion of MLC1 in a mlc1 cKO demonstrated that MyoA and MyoC cannot be anchored to the IMC. However, parasites were still able to move and invade albeit at reduced levels.

To further investigate the functions of the proteins of the motor complex, we studied attachment capacity under shear stress and retrograde flow using mutant parasites. Our results indicate that attachment capacity was altered but retrograde membrane flow does not depend on the actomyosin-system.