MICAL-mediated oxidation of actin and its effects on cytoskeletal and cellular dynamics

Abstract

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.

Keywords: MICAL1; MICAL2; MICAL3; MsrB; SelR; plexin; rab; semaphorin.

FIGURE 1

MICAL family proteins: domain organization and allosteric interaction. (A) Domain organization of Drosophila Mical and human/mammalian MICAL-1, MICAL-2, and MICAL-3. All MICALs contain the N-terminal flavoprotein monooxygenase (Redox) domain (green), followed by a calponin homology (CH) domain (orange), a LIM domain (blue), a proline-rich region (pink), and an ERM alpha (α)-like domain (red). These domains are linked by regions of variable length (//). The FAD binding (GxxGxxG, DG, and GD) motifs in the Redox domain are denoted in purple. The two Rab binding (RBD) regions (dark green) and plexin binding (PIR) region (purple) within the ERM α-like domain are also shown. Sites of phosphorylation by Abl kinase are shown as yellow circles [Y (tyrosine)], while those of PAK1 kinase are in dark blue circles [S (Serine)]. It is notable that many annotated cDNAs for MICAL-2 do not contain the C-terminal ERM α-like region, but the MICAL-2 genomic locus includes an ERM α-like region that is similar to MICAL-1 and MICAL-3. This region has independently been called MICAL-CL and Ebitein and is denoted here with dashed lines. (B) Full-length MICAL family members have been found to exist in autoinhibited (inactive) forms. In the autoinhibited form, the C-terminus ERM α-like domain of the MICAL’s folds in antiparallel fashion towards the N-terminus, and interacts with the Redox and LIM domains to inhibit the Redox enzymatic activity of the MICALs.

FIGURE 2

The MICAL’s activity and its effects on actin dynamics. (A–C) MICALs posttranslationally modify specific methionine (Met) residues in F-actin (red), which triggers F-actin disassembly (A). More specifically, MICALs bind to F-actin [(B), red arrow)] and in the presence of their coenzyme, NADPH (C), selectively and stereospecifically oxidize (O) actin’s Met44 and Met47 in the R-conformation [(C), arrowhead]. This oxidation of Met44 and Met47 [see (B), red] occurs in the D-loop, at the pointed end of individual actin filament subunits, which disrupts the interprotomers interactions in F-actin and leads to their rapid disassembly. (D) MICAL-mediated F-actin disassembly is regulated by other proteins. From top and clockwise: following MICALs’ oxidation of F-actin to generate MICAL-oxidized (Mox) F-actin, MICAL-triggered F-actin disassembly is enhanced by other proteins, including cofilin, profilin, and INF2. The Mox-G-actin that is formed does not readily re-polymerize even in the presence of profilin, formins, and Ena/VASP. Yet, Mox-G-actin is reduced specifically by selective methionine sulfoxide reductases (MsrBs/SelRs), and this G-actin can then re-polymerize normally. In this way, MICALs and MsrBs/SelRs create a reversible system for Redox regulation of actin dynamics in cells.

FIGURE 3

Regulation of the MICAL’s activity. (A) Plexins and GTP-bound Rab proteins bind the MICAL’s ERM α-like domain to relieve the MICAL’s autoinhibition. PAK1 kinases also play a role in relieving the MICAL’s autoinhibition by phosphorylating residues within the MICAL’s C-terminus. (B) Rabs and Coronin 1C are involved in the MICAL’s translocation and positioning. Myosin15 is involved in moving the MICAL’s into a new region to propagate its F-actin disassembly effects.

FIGURE 4

Cellular behavior controlled by the MICAL’s activity contributes to multiple functions and dysfunctions in numerous tissues. Modified from (Alto and Terman, 2018).