Cytoplasmic γ-actin and tropomodulin isoforms link to the sarcoplasmic reticulum in skeletal muscle fibers

T he sarcoplasmic reticulum (SR) serves as the Ca(2+) reservoir for muscle contraction. Tropomodulins (Tmods) cap filamentous actin (F-actin) pointed ends, bind tropomyosins (Tms), and regulate F-actin organization. In this paper, we use a genetic targeting approach to examine the effect of Tmod1 deletion on the organization of cytoplasmic gamma-actin (gamma(cyto)-actin) in the SR of skeletal muscle. In wild-type muscle fibers, gamma(cyto)-actin and Tmod3 defined an SR microdomain that was distinct from another Z line-flanking SR microdomain containing Tmod1 and Tmod4. The gamma(cyto)-actin/Tmod3 microdomain contained an M line complex composed of small ankyrin 1.5 (sAnk1.5), gamma(cyto)-actin, Tmod3, Tm4, and Tm5NM1. Tmod1 deletion caused Tmod3 to leave its SR compartment, leading to mislocalization and destabilization of the Tmod3-gamma(cyto)-actin-sAnk1.5 complex. This was accompanied by SR morphological defects, impaired Ca(2+) release, and an age-dependent increase in sarcomere misalignment. Thus, Tmod3 regulates SR-associated gamma(cyto)-actin architecture, mechanically stabilizes the SR via a novel cytoskeletal linkage to sAnk1.5, and maintains the alignment of adjacent myofibrils.