An anchor for motoring

An anchor for motoring oxoplasma 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. Immuno-precipitation 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. T Arg links actin and microtubules ctin both pushes out the membrane of moving cells and links to adhesion sites to give those cells traction. But actin does not operate alone. Microtubules (MTs) are targeted to the front of the cell to deliver signals that promote actin polymer-ization and thus give moving cells directionality. Now, Miller et al. (page 407) report that Arg, a nonreceptor tyrosine kinase, acts as a physical link between actin and MTs that may allow this signal transfer. Arg was known to bind and bundle actin, but the report of a MT-binding site between the two actin-binding sites is new. MTs cosediment with but are not cross-linked by Arg; cross-links do, however, form between arrays of MTs and F-actin when Arg is present. Arg staining coincides with concentrations of both F-actin and MTs at protrusive structures at the cell periphery. Cells with no Arg or only a version of Arg that cannot bind A Actin (blue) and microtubules (red) come together thanks to Arg (green). MTs show very few lamellipodial protrusions, with fewer MTs targeting any actin clusters. Arg may guide growing MTs along actin bundles or help secure MTs once they have arrived at an adhesive structure. The MTs would then deliver their signal, and Arg could target its recently identified phosphorylation substrate p190RhoGAP, resulting in less Rho activity and thus more protrusion. ᭿ This inner membrane complex lies just underneath the plasma membrane. Myosin-A and perhaps short actin …