Biochemical and structural insights into the auto-inhibited state of Mical1 and its activation by Rab8
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eLife Assessment
This fundamental study addresses the regulation of the MICAL-family of actin regulators by Rab GTPases, which play a key role in directing membrane trafficking within cells. The compelling evidence explains how Rab8 family members bind at two sites to allosterically regulate MICAL1, and relieve an auto-inhibited state unable to bind actin. This study lays the basis for further progress in understanding membrane trafficking and cytoskeleton dynamics in eukaryotic cells.
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Abstract
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.
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eLife Assessment
This fundamental study addresses the regulation of the MICAL-family of actin regulators by Rab GTPases, which play a key role in directing membrane trafficking within cells. The compelling evidence explains how Rab8 family members bind at two sites to allosterically regulate MICAL1, and relieve an auto-inhibited state unable to bind actin. This study lays the basis for further progress in understanding membrane trafficking and cytoskeleton dynamics in eukaryotic cells.
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Reviewer #1 (Public review):
The manuscript describes comprehensive structure-function studies combining structural studies, Alphafold2-based modelling, and extensive structural validation by mutagenesis and biochemical experiments. Consequently, a sophisticated activation mechanism of Mical1 as a representative of the MICAL family is elucidated at the molecular level. Since MICAL proteins are important regulators of membrane trafficking and cytoskeleton dynamics, the study is of high relevance for many groups. Structural data are of high quality, the modelling data appear to be sound, and the subsequent biochemical analyses are carried out in great detail, yielding a complete story. I have little to criticize on this beautiful work.
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Reviewer #2 (Public review):
Summary:
Rai and coworkers have studied the regulation of the MICAL-family of actin regulators by Rab 8 family GTPases. Their work uses a combination of structural biology, biochemistry, and modelling approaches to identify the regions and specific residues interacting with Rabs and understand the consequences of MICAL1 regulation. The study extends previous work on individual domains by incorporating analysis of the full-length MICAL1 protein and provides compelling evidence for allosteric regulation by Rab binding to two low and high-affinity regulatory sites.
Strengths:
Excellent biochemical and structural analysis.
Weaknesses:
Additional data to test the model for Rab regulation of MICAL1 in the actin-pelleting assay would enhance the study.
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