Structural Basis of the LARP4-Filamin A Interaction and Competition with Integrin β7 Tails.

Filamin A (FLNA) is an actin cross-linking protein that connects multiple transmembrane receptors and cytosolic signaling proteins to regulate cell shape, motility, and signaling. Our previous report has shown that FLNA interacts directly with the La-related protein 4 (LARP4) and this interaction is essential for cell migration. Here, using the x-ray crystallography and protein-protein interaction studies, we investigated the molecular basis of LARP4 binding to FLNA. We described the high-resolution structure of the FLNA immunoglobulin-like repeat 21 (R21) and its complex with the LARP4 peptide. The FLNA-binding site in LARP4 is localized between Ala269 and Asn281, where it forms an extended β strand that interacts with the cleft formed by β strands C and D of FLNA R21. Consistent with this structure, the A279Cfs*2 mutation found in catalogue of somatic mutations in cancer (COSMIC) database and the experimentally introduced F277A mutation both disrupt LARP4 binding to FLNA. In contrast, the COSMIC-listed N275S mutation alters LARP4 membrane localization without affecting FLNA interaction, suggesting distinct functional outcomes. Cell migration assays showed that LARP4-knockdown cells expressing FLNA-binding-deficient mutants migrated faster than those expressing wild-type LARP4. The LARP4-binding site on FLNA overlaps with the β-integrin tail-binding region, and in vitro assays revealed that LARP4 can compete with integrin β7 tails for FLNA R21 binding. These results suggest that the LARP4-FLNA interaction may regulate cell migration, at least in part, by competing with integrin tails, although this mechanism has yet to be confirmed in vivo.