Even small amounts of alpha-actinin or filamin have a significant effect on the physical properties of a solution of actin filaments, changing it from a sludgy solution to a semisolid gel. This is especially true in the case of filamin, a long and flexible molecule. One filamin dimer per 200 molecules of actin monomers is enough to create a large, continuous network. Synthetic actin gels made this way tend to preserve their form when subjected to a sudden force, but readily deform under a low, consistent pressure, the result of the links having more time to dissociate. In the digital video presented above, human glioblastoma cells (U373 line) are expressing a fusion of mCherry fluorescent protein to human alpha-actinin.
Regularly spaced, periodic structures observed to traverse the short axis of actin stress fibers are often referred to as dense regions or dense bodies, and are thought to be the cellular version of structural and functional analogs of sarcomere Z-disks. A core component of dense regions is alpha-actinin, which has been demonstrated to associate with other molecules, including LIM and PDZ domain proteins.
A member of the spectrin superfamly, alpha-actinin is an actin crosslinking protein that forms antiparallel homodimers in a rod-like structure with one actin-binding domain on each side of the rod. In addition to its binding to actin, alpha-actinin also associates with a number of other cytoskeletal proteins including titin, zyxin, vinculin, and alpha-catenin, as well as cytoplasmic signaling proteins, such as phosphatidylinositol 3-kinase, Rho effector kinase, and the beta-integrins. In the digital video presented above, a normal rabbit kidney epithelial cell (RK-13 line) expressing a fusion of mCherry fluorescent protein with human alpha-actinin is observed traversing the coverslip in a live-cell imaging chamber using fluorescence and differential interference contrast (DIC) illumination. Note the high levels of labeled alpha-actinin present in the lamellipodia, which continually undergo wave-like motions.
