Visualization of the actin cytoskeleton: different F-actin-binding probes tell different stories.

The actin cytoskeleton is necessary for cell viability and plays crucial roles in cell motility, endocytosis, growth, and cytokinesis. Hence visualization of dynamic changes in F-actin distribution in vivo is of central importance in cell biology. This has been accomplished by the development of fluorescent protein fusions to actin itself or to various actin-binding proteins, actin cross-linking proteins, and their respective actin-binding domains (ABDs). Although these protein fusions have been shown to bind to F-actin in vivo, we show that the fluorescent protein used for visualization changes the subset of F-actin labeled by an F-actin ABD probe. Further, different amino acid linkers between the fluorescent protein and ABD induced a similar change in localization. Although different linkers and fluorescent proteins can alter the subset of actin bound by a particular ABD, in most cases, the fusion protein did not label all of a cell's F-actin all of the time. Even LimEΔcoil and GFP-actin, which have been used extensively for cytoskeletal visualization, were highly variable in the subsets of actin that they labeled. Lifeact, conversely, clearly labeled cortical F-actin as well as F-actin in the anterior pseudopods of motile cells and in macropinocytotic cups. We conclude that Lifeact most accurately labels F-actin and is the best currently available probe for visualization of dynamic changes in F-actin networks.