Mechanics of actin networks crosslinked with mutant human $\alpha$-actinin-4

Globular actin can be polymerized {\em in vitro} to form F-actin in the presence of various binding proteins. These networks often exhibit dramatic nonlinear rheological response to imposed strains. We study the rheological properties of F-actin networks crosslinked with human $\alpha$-actinin-4. A single genetic mutation of the $\alpha$-actinin-4 protein is associated with focal and segmented glomerulosclerosis (FSGS), a genetic disorder which leads to renal failure. Mechanically, the mutant crosslinker has an increased binding strength compared to the wild type. We will show that human $\alpha$-actinin-4, displays a unique stiffening response. Moreover, we also demonstrate that a single point mutation dramatically effects the inherent relaxation time of the crosslinked network.