Boczkowska et al. (2026). Nature Communications. doi: 10.1038/s41467-026-74810-6
…facing actin in the structures (Supplementary Fig. 9). Methods Proteins for cryo-EM analysis Skeletal α-actin54 and Tpm1.155 were purified from fresh hind leg rabbit muscle tissue purchased from Pel-Freeze Biologicals. Full-length human Lmod2 was obtained as we have previously described18. Briefly, the gene was cloned between the NdeI and SapI sites of vector pTYB1 (NEB), which encodes a chitin-binding domain for…
Abstract
In non-muscle cells, actin filaments exhibit variable lengths and rapid turnover, with subunits adding primarily at the barbed end. The situation is strikingly different in striated muscle sarcomeres, where despite rapid turnover, actin thin filaments exhibit uniform length and exchange subunits primarily at the pointed end. This filament length uniformity is tightly regulated by several proteins, including the molecular ruler nebulin in skeletal muscle and the barbed- and pointed-end capping proteins CapZ and tropomodulin (Tmod) in both skeletal and cardiac muscles. Recent studies in cells and animal models have identified leiomodin-2 (Lmod2) as an additional regulator proposed to promote pointed-end elongation to maintain thin filament length. This activity would make leiomodin the only known eukaryotic factor to drive pointed-end elongation, yet its molecular mechanism remains unresolved. Here, we present a series of cryo-electron microscopy structures that support a stepwise elongation mechanism in which two Lmod2 molecules alternate at the pointed end while recruiting actin monomers. These findings establish the molecular basis of pointed-end elongation in muscle sarcomeres and provide a framework for understanding mutations in Lmod2 that cause dilated cardiomyopathy.