Cardiomyopathy‐associated and basic residue mutations in myopalladin alter actin binding, bundling, and structural stability

Myopalladin (MYPN) is a striated muscle‐specific protein essential for sarcomere integrity and actin filament organization. The Ig3 domain of MYPN mediates direct binding and bundling of filamentous actin (F‐actin), and several cardiomyopathy (CM)‐associated mutations cluster within this domain. To elucidate the molecular basis of MYPN‐actin interactions, we performed alanine‐scanning mutagenesis of conserved basic residues and evaluated CM‐linked variants of the Ig3 domain using co‐sedimentation assays. These mutations impaired actin binding and abolished bundling activity, despite preserving secondary structure in most cases. Circular dichroism revealed that the P961L variant adopts a more open, partially unfolded β‐sandwich conformation, distinct from the wild type (WT), which correlates with its aggregation and mislocalization in Drosophila cardiomyocytes. In vivo expression of full‐length WT and mutant MYPN in Drosophila muscle confirmed Z‐disc localization for WT, while R955W and P961L mutants formed aberrant clusters. Biochemical assays demonstrated that MYPN Ig3 promotes actin polymerization and bundling even under non‐polymerizing conditions, and its interaction with actin is electrostatically driven but calcium‐independent. Sedimentation equilibrium analysis confirmed that MYPN Ig3 functions as a monomer, suggesting that bundling arises from dual actin‐binding surfaces rather than from self‐oligomerization. These findings establish the Ig3 domain as a structurally robust, actin‐regulatory module and identify disruption of MYPN–actin interactions as a pathogenic mechanism in CM. Our study provides the first direct evidence linking Ig3 domain mutations to impaired actin dynamics and sarcomere disorganization, offering mechanistic insight into MYPN‐associated cardiomyopathies.