TgGAP50 (green) anchors TgMyoA (red) in the inner membrane.

Toxoplasma gondii, a parasite that can cause blindness and brain damage in immunocompromised individuals, spreads around the body and penetrates into cells using a bizarre spinning and corkscrewing motility. This motility is not dependent on any easily visible structure, such as a flagellum or lamellipodia, and the cell moves without changing shape. Now, Gaskins et al. (page 383) report the identification of TgGAP45 and TgGAP50, two proteins that may anchor myosin so that the motor can drive this unique form of motility.

The authors began by isolating and raising antibodies to the inner membrane, where the myosin-A (TgMyoA) responsible for motility is thought to hang out. They found TgGAP45, a protein of unknown function. Immunoprecipitation of TgGAP45 yielded TgMyoA, its associated light chain, and TgGAP50, an integral membrane protein. The TgGAP45, TgMyoA, and light chain assemble in the cytoplasm and then apparently meet the TgGAP50 in the inner membrane complex.

This inner membrane complex lies just underneath the plasma membrane. Myosin-A and perhaps short actin filaments lie between the two membranes, with actin filaments anchored to the extracellular substrate via an adhesin. Gaskins et al. suggest that TgMyoA attaches to both the actin and, via TgGAP50, to the inner membrane complex. A stable cytoskeletal matrix known to exist on the other side of the inner membrane may immobilize TgGAP50 itself. TgMyoA may either walk along the actin filament, pulling the cell body closer to any adhesion sites, or it may ride forward near the front of any growing actin filaments. Exactly how this leads to overall cell movement, especially movement that is directional, is not yet clear. ▪