N-Terminal Fragment of Cardiac Myosin Binding Protein-C Increases the Duration of Actin-Myosin Interaction.

Cardiac myosin binding protein-C (cMyBP-C) located in the C-zone of myocyte sarcomere is involved in the regulation of myocardial contraction. Its N-terminal domains C0, C1, C2, and the m-motif between C1 and C2 can bind to the myosin head and actin of the thin filament and affect the characteristics of their interaction. Measurements using an optical trap showed that the C0-C2 fragment of cMyBP-C increases the interaction time of cardiac myosin with the actin filament, while in an in vitro motility assay, it dose-dependently reduces the sliding velocity of actin filaments. Thus, it was found that the N-terminal part of cMyBP-C affects the kinetics of the myosin cross-bridge.

Possible Cause of Nonlinear Tension Rise in Activated Muscle Fiber during Stretching.

Tension in contracting muscle fiber under conditions of ramp stretching rapidly increases, but after reaching a critical stretch Pc sharply decreases. To find out the cause of these changes in muscle fiber tension, we stopped stretching before and after reaching Pc and left the fiber stretched for 50 msec. After rapid tension drop, the transient tension rise not accompanied by fiber stiffness increase was observed only in fibers heated to 25°C and stretched to Pc. Under other experimental conditions, this growth was absent. We suppose that stretch of the fiber to Pc induces transition of stereo-specifically attached myosin heads to pre-power stroke state and when the stretching is stopped, they make their step on actin and generate force. When the tension reaches Pc, all stereospecifically attached myosin heads turn out to be non-stereospecifically, or weakly attached to actin, and are unable to make the force-generating step.