Properties and catalytic activities of MICAL1, the flavoenzyme involved in cytoskeleton dynamics, and modulation by its CH, LIM and C-terminal domains

MICAL1 is a cytoplasmic 119 kDa protein participating in cytoskeleton dynamics through the NADPH-dependent oxidase and F-actin depolymerizing activities of its N-terminal flavoprotein domain, which is followed by calponin homology (CH), LIM domains and a C-terminal region with Pro-, Glu-rich and coiled-coil motifs. MICAL1 and truncated forms lacking the C-terminal, LIM and/or CH regions have been produced and characterized. The CH, LIM and C-terminal regions cause an increase of Km,NADPH exhibited by the NADPH oxidase activity of the flavoprotein domain, paralleling changes in the overall protein charge. The C-terminus also determines a ∼10-fold decrease of kcat, revealing its role in establishing an inactive/active conformational equilibrium, which is at the heart of the regulation of MICAL1 in cells. F-actin lowers Km,NADPH (10-50 μM) and increases kcat (10-25 s−1) to similar values for all MICAL forms. The apparent Km,actin of MICAL1 is ∼10-fold higher than that of the other forms (3-5 μM), reflecting the fact that F-actin binds to the flavoprotein domain in the MICAL's active conformation and stabilizes it. Analyses of the reaction in the presence of F-actin indicate that actin depolymerization is mediated by H2O2 produced by the NADPH oxidase reaction, rather than due to direct hydroxylation of actin methionine residues.