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

Abstract

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.

Keywords: F-actin; actin-binding protein; cytoskeleton; fluorescent protein.