Abstract
Cardiac sarcomere assembly is a highly orchestrated process requiring integration between intracellular contractile machinery and extracellular adhesions. While α-actinin-2 (ACTN2) is well known for its structural role at the cardiac Z-disc, the sarcomere border, the function of the "non-muscle" paralog α-actinin-1 (ACTN1) in cardiac myocytes remains unclear. Using human induced pluripotent stem cell-derived cardiac myocytes (hiCMs), we demonstrate that siRNA-mediated depletion of ACTN1 disrupts sarcomere assembly, and that exogenous re-introduction of ACTN1 but not ACTN2 restores assembly, revealing non-redundant functions. Unlike ACTN2, ACTN1 localized predominantly to cardiac myocyte focal adhesions, and was required for adhesion enlargement during sarcomere assembly, suggesting ACTN1 but not ACTN2 is required for adhesion maturation. Live-cell imaging of vinculin dynamics showed decreased stability of adhesion-associated vinculin in ACTN1-deficient cells, whereas paxillin dynamics were unaffected. These results suggest that ACTN1 stabilizes focal adhesions to promote effective force transmission during sarcomere assembly.