Bending stiffness of Toxoplasma gondii actin filaments.

Actin is essential for the survival and pathogenicity of the Apicomplexan parasite Toxoplasma gondii, where it plays essential functions in cargo transport, invasion, egress, and organelle inheritance. Recent work has shown that, unlike vertebrate skeletal muscle actin, purified T. gondii actin filaments (TgAct1) can undergo rapid treadmilling, due to large differences in the barbed- and pointed-end critical concentrations, rapid subunit dissociation from filament ends, and a rapid nucleotide exchange rate constant from free monomers. Previous structural analysis suggested that the unique assembly properties of TgAct1 filaments may be a functional consequence of reduced contacts between the DNAse-1-binding loop (D-loop) of a filament subunit and its adjacent, long-axis subunit neighbor. Because the D-loop makes stabilizing interactions between neighboring subunits, it has been implicated in regulating the mechanical properties of actin filaments. In this study, we measured the bending persistence length (L(B)) of TgAct1 filaments and the filament length distribution. We found that despite compromised intersubunit D-loop contacts, TgAct1 filaments have similar bending stiffness and thermodynamic stability as vertebrate actin filaments. Analysis of published cryo-EM image density maps indicates that TgAct1 filaments retain a stabilizing intersubunit salt bridge between E168 and K62 and reveals visible density between Y167 and S61 of adjacent filament subunits, consistent with a conserved cation binding site proximal to the D-loop, as initially identified in vertebrate skeletal muscle actin filaments. These results favor a mechanism in which weak D-loop interactions compromise TgAct1 subunit incorporation at filament ends, while minimally affecting overall subunit interactions within filaments.