Troponin I is the inhibitory subunit of troponin, the thin filament regulatory complex which confers calcium-sensitivity to striated muscle actomyosin ATPase activity. Binds to actin and tropomyosin. Highest levels observed in human skeletal muscle (e.g. gastrocnemious muscle), differentiated cultures of primary human muscle cells and rhabdomyosarcoma cells cultured in low serum medium. Expressed in C2 muscle cell myoblasts and myotubes. Belongs to the troponin I family.