Unusual dynamics of the divergent malaria parasite Act1 actin filament.

Gliding motility and host cell invasion by the apicomplexan parasite (), the causative agent of malaria, is powered by a macromolecular complex called the glideosome that lies between the parasite plasma membrane and the inner membrane complex. The glideosome core consists of a single-headed class XIV myosin MyoA and a divergent actin Act1. Here we use total internal reflection fluorescence microscopy to visualize growth of individual unstabilized Act1 filaments as a function of time, an approach not previously used with this actin isoform. Although Act1 was thought to be incapable of forming long filaments, filaments grew as long as 30 µm. Polymerization occurs via a nucleation-elongation mechanism, but with an ∼4 µM critical concentration, an order-of-magnitude higher than for skeletal actin. Protomers disassembled from both the barbed and pointed ends of the actin filament with similar fast kinetics of 10 to 15 subunits/s. Rapid treadmilling, where the barbed end of the filament grows and the pointed end shrinks while maintaining an approximately constant filament length, was visualized near the critical concentration. Once ATP has been hydrolyzed to ADP, the filament becomes very unstable, resulting in total dissolution in <40 min. Dynamics at the filament ends are suppressed in the presence of inorganic phosphate or more efficiently by BeF A chimeric Act1 with a mammalian actin D-loop forms a more stable filament. These unusual dynamic properties distinguish Act1 from more canonical actins, and likely contribute to the difficultly in visualizing Act1 filaments in the parasite.