| α‑Cardiac actin, encoded by ACTC1, is the predominant sarcomeric actin isoform in developing and adult heart muscle and forms the core of thin filaments in cardiac myofibrils, where it organizes with myosin, tropomyosin, and the cardiac troponin complex to generate force during excitation–contraction coupling. The protein belongs to the α‑actin subgroup of the actin family and shares high overall sequence conservation with other muscle and nonmuscle actins, but the N‑terminal region carries isoform-specific residues that influence interactions with myosin heads, actin-binding proteins, and Z‑line components, and this region constitutes a major antigenic and regulatory interface in cardiac myocytes. Polymerization of α‑cardiac actin from globular (G‑actin) to filamentous (F‑actin) form generates the helical thin filaments that extend from Z‑discs toward the sarcomere center and provide the scaffold for myosin cross‑bridge cycling; within this context, α‑cardiac actin interacts with β‑myosin heavy chain and the Ca²⁺‑regulated troponin–tropomyosin complex so that small changes in actin conformation or binding energetics translate into altered Ca²⁺ sensitivity and maximal contractile force. Allosteric coupling within the actin filament allows amino-acid substitutions located away from direct myosin or regulatory protein contact sites to modify long‑range dynamics and thereby shift the balance between calcium sensitivity and force-generation pathways, and pathogenic ACTC1 variants such as T126I and S271F exemplify how altered actin allostery can drive either hypo‑contractile or hyper‑contractile phenotypes at the sarcomere level. α‑Cardiac actin accounts for the majority of myofibrillar actin in human donor hearts and represents the dominant sarcomeric isoform in embryonic heart and early skeletal muscle, while in the mature human heart it coexists with smaller amounts of α‑skeletal actin whose expression increases in pressure overload and hypertrophic remodeling. The ACTC1 gene participates in Rho–SRF–MRTF–regulated transcriptional networks that coordinate cytoskeletal gene expression with mechanical load, and α‑cardiac actin treadmilling influences the nucleo–cytoplasmic shuttling of myocardin‑related transcription factors, linking sarcomeric actin dynamics to adaptive changes in cardiomyocyte gene programs during growth and stress. Germline ACTC1 mutations associate with familial hypertrophic and dilated cardiomyopathies and with structural defects such as atrial septal defect and left ventricular noncompaction, where altered interactions between α‑cardiac actin and β‑myosin or the troponin–tropomyosin regulatory complex result in changes in contractile force, Ca²⁺ responsiveness, and ventricular geometry that can manifest as systolic or diastolic dysfunction. |
