UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Mechanical integrity of myosin thick filaments of airway smooth muscle in vitro: effects of phosphoryation of the regulatory light chain Ip, Kelvin


Background and aims: It is known that smooth muscle possesses substantial mechanical plasticity in that it is able to adapt to large changes in length without compromising its ability to generate force. It is believed that structural malleability of the contractile apparatus underlies this plasticity. There is strong evidence suggesting that myosin thick filaments of the muscle are relatively labile and their length in vivo is determined by the equilibrium between monomeric and filamentous myosin. The equilibrium in turn is governed by the state of phosphorylation of the 20-kD regulatory myosin light chain (MLC20, or RLC). It is known that phosphorylation of the myosin light chain favors formation of the filaments; it is not known how the light chain phosphorylation affects the lability of the filaments. The major aim of this thesis was to measure the mechanical integrity of the filaments formed from purified myosin molecules from bovine airway smooth muscle, and to determine whether the integrity was influenced by phosphorylation of the myosin light chain. Methods: Myosin was purified from bovine trachealis to form filaments, in ATP containing zero-calcium solution during a slow dialysis that gradually reduced the ionic strength. Sufficient myosin light chain kinase and phosphatase, as well as calmodulin, were retained after the myosin purification and this enabled phosphorylation of RLC within 20-40 s after addition of calcium to the filament suspension. The phosphorylated and non-phosphorylated filaments were then partially disassembled by ultrasonification. The extent of filament disintegration was visualized and quantified by atomic force microscopy. Results: RLC phosphorylation reduced the diameter of the filaments and rendered the filaments more resistant to ultrasonic agitation. Electron microscopy revealed a similar reduction in filament diameter in intact smooth muscle when the cells were activated. Conclusion: Our results suggest that RLC phosphorylation is a key regulatory step in modifying the structural properties of myosin filaments in smooth muscle, where formation and dissolution of the filaments are required in the cells’ adaptation to different cell length.

Item Media

Item Citations and Data


Attribution-NonCommercial-NoDerivatives 4.0 International