Sensing actin dynamics: structural basis for G-actin-sensitive nuclear import of MAL

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Cytoskeletal actin dynamics coordinate with gene expression for cellular processes like migration and differentiation.
• The actin-binding transcriptional coactivator MAL (also known as MRTF-A/MKL1/BSAC) links G-actin levels and Rho GTPase signaling to the transcription factor SRF.

Purpose of the Study:

• To elucidate the molecular and structural basis of actin-regulated nucleocytoplasmic shuttling of MAL.
• To investigate the mechanism by which G-actin binding affects MAL's interaction with importin-α.

Main Methods:

• X-ray crystallography to determine the structure of MAL RPEL domain complexes.
• Biochemical assays to study protein-ligand interactions.
• Cell biological experiments to visualize MAL localization and function.

Main Results:

• Identified a classical bipartite nuclear localization signal (NLS) in MAL's N-terminal 'RPEL' domain.
• Structural analysis revealed MAL's NLS binds to importin-α's major and minor NLS binding sites.
• Crystal structure of MAL RPEL domain complexed with G-actin shows NLS residues adopt a helical conformation upon actin binding, precluding importin-α recognition.

Conclusions:

• MAL possesses a classical bipartite NLS within its RPEL domain, mediating nuclear import via importin-α.
• G-actin binding induces a conformational change in MAL's NLS, inhibiting its interaction with importin-α and thus regulating nucleocytoplasmic shuttling.
• This mechanism provides insight into how actin dynamics control gene expression reprogramming.