Actin and its dynamic structural remodelings are involved in multiple cellular functions, including maintaining cell shape and integrity, cytokinesis, motility, navigation, and muscle contraction. Many actin-binding proteins regulate the cytoskeleton to facilitate these functions. Recently, actin’s post-translational modifications (PTMs) and their importance to actin functions have gained increasing recognition. The MICAL family of proteins has emerged as important actin regulatory oxidation-reduction (Redox) enzymes, influencing actin’s properties both in vitro and in vivo. MICALs specifically bind to actin filaments and selectively oxidize actin’s methionine residues 44 and 47, which perturbs filaments’ structure and leads to their disassembly. This review provides an overview of the MICALs and the impact of MICAL-mediated oxidation on actin’s properties, including its assembly and disassembly, effects on other actin-binding proteins, and on cells and tissue systems.

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties—both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

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Actin filaments are the major component of cellular substructures involved in cell motility and cell sensing, depending on the cues from the microenvironment. Actin’s diverse functions are enabled by its transitions from a monomeric (G‐actin) to filamentous (F‐actin) form, which in turn can be converted into higher order structures, like bundles, with the help of various actin‐crosslinking proteins. The mechanism of actin bundles formation has been previously reported, but how they disassemble is understood less well. Using real‐time solution‐based assays and purified proteins we try to unravel the specific mechanism by which the MICAL family of monooxygenase induces the disassembly of bundled actin. These experiments were further supported by our in vivo studies using a model cellular system. We showed that the rate of disassembly is enhanced upon addition of cofilin, a well‐known F‐actin severing protein. Overall, our results shed new light on the mechanism of actin bundle disassembly and its direct impact on cellular remodeling.

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