Biochemical and structural insights into the auto-inhibited state of Mical1 and its activation by Rab8

Mical1 regulates F-actin dynamics through the reversible oxidation of actin, a process controlled by its interactions with various proteins. Upon binding to Rab8 family members, Mical1 links endosomes to the cytoskeleton, promoting F-actin disassembly. In the absence of Rab, Mical1 exists in an auto-inhibited state, but its biochemical characterization remains incomplete. Our study reveals that the N-terminal MO-CH-LIM domains of Mical1 form an intramolecular complex with its C-terminal bMERB domain. Mutational analysis, guided by the AlphaFold2 model, identifies critical residues at the binding interface. Additionally, we demonstrate that full-length Mical1 binds to Rab8 in a 1:2 stoichiometry, thereby releasing auto-inhibition. Through structure-based mutational studies, we uncover allostery between the N and C-terminal Rab binding sites. Notably, Rab binding at the high-affinity C-terminal site precedes binding at the N-terminal site, suggesting a sequential binding mode. These findings elucidate how Rab8 binding releases the MO-CH-LIM domains from the Mical1 bMERB domain, facilitating interactions with other proteins and the actin cytoskeleton, thereby modulating actin dynamics.