Self-assembly of smooth muscle myosin filaments: adaptation of filament length by telokin and Mg·ATP

Purification of Myosin from Bovine Tracheal Smooth Muscle, Filament Formation and Endogenous Association of Its Regulatory Complex

Dynamic regulation of myosin filaments is a crucial factor in the ability of airway smooth muscle (ASM) to adapt to a wide length range. Increased stability or robustness of myosin filaments may play a role in the pathophysiology of asthmatic airways. Biochemical techniques for the purification of myosin and associated regulatory proteins could help elucidate potential alterations in myosin filament properties of asthmatic ASM. An effective myosin purification approach was originally developed for chicken gizzard smooth muscle myosin. More recently, we successfully adapted the procedure to bovine tracheal smooth muscle. This method yields purified myosin with or without the endogenous regulatory complex of myosin light chain kinase and myosin light chain phosphatase. The tight association of the regulatory complex with the assembled myosin filaments can be valuable in functional experiments. The purification protocol discussed here allows for enzymatic comparisons of myosin regulatory proteins. Furthermore, we detail the methodology for quantification and removal of the co-purified regulatory enzymes as a tool for exploring potentially altered phenotypes of the contractile apparatus in diseases such as asthma.

Myosin assembly of smooth muscle: from ribbons and side polarity to a row polar helical model

After decades of debate over the structure of smooth muscle myosin filaments, it is still unclear whether they are helical, as in all other muscle types, or square in shape. In both cases bipolar building units are proposed, but the deduced cross-bridge arrangements are fundamentally different. The opposite polarity of the adjusting longitudinal rows is proposed for the helical structure, while in the case of square filaments, or myosin ribbons, only their two faces are appositively polarized. Analysis of our unpublished archival data on light meromyosin (LMM) paracrystals and myosin rod assemblies as well as the filaments themselves indicated that the rods were assembled with a 6°-7° tilt angle from the rods' longitudinal axis, in contrast to the lack of tilt in LMM, both exhibiting a 14.3 nm myosin periodicity. Optical diffraction analysis of EM images of the rod assemblies and those of intact myosin confirmed their helical architecture characterized by 28 nm residue translations, 172 nm repeats and 516 nm pitch. A detailed helical model of these filaments was elucidated with bipolar tetramer building units made of two polar trimers. The filaments elongate at their two ends in a head-to-head manner, enabling targeted cross-bridge polarity of the adjacent rows, in the form of a unique Boerdijk-Coxeter type helix, similar to that of collagen or desmin fibers, with the covalent links replaced by a head-to-head clasp.