Significance
Myosin-binding protein C (MyBP-C) is a component of myosin filaments, one of the two sets of contractile elements whose relative sliding is the basis of muscle contraction. In the heart, MyBP-C modulates contractility in response to cardiac stimulation; mutations in MyBP-C lead to cardiac disease. The mechanism by which MyBP-C modulates cardiac contraction is not understood. Using electron microscopy and a light microscopic assay for filament sliding, we demonstrate that MyBP-C binds to the other set of filaments, containing actin and the regulatory component, tropomyosin. In so doing, it displaces tropomyosin from its inhibitory position to activate actin filament interaction with myosin, promoting filament sliding. These findings provide insights into the molecular basis of heart function.
Actin plays a structural role in the cytoplasm. However, actin takes on new functions and structures in the nucleus that are poorly understood. The nuclei of the large oocytes of the frog X. laevis specifically import actin to reach high concentrations; however, it remains unclear what, if any, structural role this actin plays. Here, we use microrheological and confocal imaging techniques to probe the local architecture and mechanics of the nucleus. Our data show that actin forms a weak network that spatially organizes the nucleus by kinetically stabilizing embedded liquid-like RNA/protein bodies which are important for cell growth. In actin-disrupted nuclei, this RNA/protein droplet emulsion is destabilized, and undergoes rapid gravitational sedimentation and fusion. This suggests that gravitational stabilization of subcellular structures is an important mechanical requirement in large cells.
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