Cardiac muscle

Biology

Troponin

Materials science

Diastole

Myosin light-chain kinase

Endocrinology

Biochemistry

Protein filament

Actin

Blood pressure

Tropomyosin

Anatomy

Muscle contraction

Myosin

Cooperativity

Troponin C

Systole

Myocardial infarction

Internal medicine

Composite material

Crystallography

Medicine

Chemistry

Myosin head

Biophysics

Proceedings of the National Academy of Sciences

Significance Heartbeats rely on cyclical interactions between myosin thick and actin thin filaments orchestrated by rising and falling Ca 2+ . During systole, Ca 2+ binds to the thin filament and allows its interaction with the thick filament to produce force required for contraction. The structure of the thin filament at physiological Ca 2+ levels is unknown, which limits our understanding of the thin filament regulation by Ca 2+ . Here, we directly observe thin filament structural states along individual filaments at systolic Ca 2+ levels to show that the thin filament is activated stochastically with short-range cooperativity evident only on one strand of the thin filament. We suggest how cardiac muscle can be regulated by a narrow range of Ca 2+ levels upon transition from diastole to systole.

The structure of the native cardiac thin filament at systolic Ca <sup>2+</sup> levels

The structure of the native cardiac thin filament at systolic Ca 2+ levels

The structure of the native cardiac thin filament at systolic Ca ²⁺ levels