Comparative Analysis of Closely Related Class XI Myosins Reveals Molecular Determinants of Actin Trajectory Curvature

Left-right asymmetry in multicellular organisms is often rooted in cellular chirality emerging from asymmetric actin cytoskeleton dynamics. Although several myosins exhibit chiral actin motility in vitro, the molecular determinants regulating actin trajectory curvature remain poorly understood. We found that two closely related class XI myosins from the alga Chara braunii, CbXI-3 and CbXI-4, share ~90% sequence identity yet display strikingly different actin trajectory curvatures, providing a comparative model system. By swapping subdomains between CbXI-3 and CbXI-4, we identified the upper and lower 50-kDa subdomains (U50/L50) as major structural determinants of curvature. Within these, the CM loop in U50 and the HTH loop in L50 are key contributors. Single amino acid substitutions, particularly S388T in the CM loop of CbXI-3, markedly increased curvature, while reciprocal mutations in CbXI-4 partially reversed the phenotype. Combinatorial mutations revealed cooperative effects among multiple residues, indicating curvature is encoded by local interaction networks rather than a single site. Our findings suggest that subtle steric and electrostatic perturbations near the interface generate directional motility. This study provides molecular insight into chiral motor function and establishes a general strategy for identifying motor protein functional diversification.