Role Of Myosin Binding Proteins On The Structural Stability And Flexural Rigidity Of Thick Filaments

Abstract

Despite the fundamental role of thick filaments in muscle contraction, little is known about the mechanical behavior of these filaments and how myosin associated proteins dictate differences between muscle types. Insect flight muscle (IFM) and vertebrate cardiac muscle share common physiological properties such as their cyclical contraction for producing either a wing beat or a heart beat, as well as their reliance on a pronounced stretch activation response to produce oscillatory power. We used atomic force microscopy (AFM) to study the morphological and biomechanical properties of native thick filaments from age-matched normal (+/+) and mutant (t/t) mice heart lacking cardiac myosin binding protein C (cMyBPC) and from IFM of normal (fln+) and mutant (fln0) Drosophila lacking flightin. AFM images of these filaments were evaluated with an automated analysis algorithm that identified filament position and shape. The t/t thick filament length (1.48 ± 0.02 μm) was significantly (P < 0.01) shorter than +/+ (1.56 ± 0.02 μm). To determine if cMyBP-C contributes to the mechanical properties of thick filaments, we used statistical polymer chain mechanics to calculate a per filament specific persistence length (PL), an index of flexural rigidity directly proportional to Young's modulus. PL in the t/t (373 ± 62 μm) was significantly lower than +/+ (639 ± 101 μm). Accordingly the Young's modulus of t/t thick filaments was approximately 60% of +/+. Thick filaments from newly eclosed fln0 IFM have longer contour length (3.90±1.33 μm) than fln+ filaments from same age flies (3.00±0.38 μm), and a PL less than half that of IFM filaments from fln+ flies. These results provide a new understanding for the critical role of myosin binding proteins in defining normal cardiac and IFM output by sustaining force and muscle stiffness.