SummaryThe mitotic spindle in eukaryotic cells is composed of a bipolar array of microtubules (MTs) and associated proteins that are required during mitosis for the correct partitioning of the two sets of chromosomes to the daughter cells. In addition to the well-established functions of MT-associated proteins (MAPs) and MT-based motors in cell division, there is increasing evidence that the F-actin-based myosin motors are important mediators of F-actin-MT interactions during mitosis. Here, we report the functional characterization of the long-tailed class-1 myosin myosin-1C from Dictyostelium discoideum during mitosis. Our data reveal that myosin-1C binds to MTs and has a role in maintenance of spindle stability for accurate chromosome separation. Both myosin-1C motor function and taildomain-mediated MT-F-actin interactions are required for the cell-cycle-dependent relocalization of the protein from the cell periphery to the spindle. We show that the association of myosin-1C with MTs is mediated through the tail domain. The myosin-1C tail can inhibit kinesin motor activity, increase the stability of MTs, and form crosslinks between MTs and F-actin. These data illustrate that myosin-1C is involved in the regulation of MT function during mitosis in D. discoideum.

The β-myosin heavy chain expressed in ventricular myocardium and the myosin heavy chain (MyHC) in slow-twitch skeletal soleus muscle type-I fibers are both encoded by MYH7. Thus, these myosin molecules are deemed equivalent. However, some reports suggested variations in the light chain composition between soleus and ventricular myosin, which could influence functional parameters such as maximum velocity of shortening. To test for functional differences of the actin gliding velocity on immobilized myosin molecules we made use of the in vitro motility assay.We found that ventricular myosin moved actin filaments with approx. 0.9 μm/s significantly faster than soleus myosin (0.3 μm/s). Unregulated actin filaments are not the native interaction partner of myosin and are believed to slow down movement. Yet, using native thin filaments purified from soleus or ventricular tissue, the gliding velocity of soleus and ventricular myosin remained significantly different. When comparing the light chain composition of ventricular and soleus β-myosin a difference became evident. Soleus myosin contains not only the “ventricular” essential light chain (ELC) MLC1sb/v, but also an additional longer and more positively charged MLC1sa. Moreover, we revealed that on a single muscle fiber level, a higher relative content of MLC1sa was associated with significantly slower actin gliding.We conclude that the ELC MLC1sa decelerates gliding velocity presumably by a decreased dissociation rate from actin associated with a higher actin affinity compared to MLC1sb/v. Such ELC/actin interactions might also be relevant in vivo as differences between soleus and ventricular myosin persisted when native thin filaments were used.SummaryCompared to the “ventricular” essential myosin light chain MLC1sb/v, the longer and more positively charged MLC1sa present in slow-twitch soleus muscle fibers decelerates actin filament gliding on β-myosin molecules presumably by a decreased dissociation rate from actin filaments.

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The β-myosin heavy chain expressed in ventricular myocardium and the myosin heavy chain (MyHC) in slow-twitch skeletal Musculus soleus (M. soleus) type-I fibers are both encoded by MYH7. Thus, these myosin molecules are deemed equivalent. However, some reports suggested variations in the light chain composition between M. soleus and ventricular myosin, which could influence functional parameters, such as maximum velocity of shortening. To test for functional differences of the actin gliding velocity on immobilized myosin molecules, we made use of in vitro motility assays. We found that ventricular myosin moved actin filaments with ∼0.9 µm/s significantly faster than M. soleus myosin (0.3 µm/s). Filaments prepared from isolated actin are not the native interaction partner of myosin and are believed to slow down movement. Yet, using native thin filaments purified from M. soleus or ventricular tissue, the gliding velocity of M. soleus and ventricular myosin remained significantly different. When comparing the light chain composition of ventricular and M. soleus β-myosin, a difference became evident. M. soleus myosin contains not only the “ventricular” essential light chain (ELC) MLC1sb/v, but also an additional longer and more positively charged MLC1sa. Moreover, we revealed that on a single muscle fiber level, a higher relative content of MLC1sa was associated with significantly slower actin gliding. We conclude that the ELC MLC1sa decelerates gliding velocity presumably by a decreased dissociation rate from actin associated with a higher actin affinity compared to MLC1sb/v. Such ELC/actin interactions might also be relevant in vivo as differences between M. soleus and ventricular myosin persisted when native thin filaments were used.

Introduction: The muscle myosin uses energy derived from ATP hydrolysis to perform mechanical work and move actin filaments, thereby generating force. The goal of this study was to investigate the force production by isolated myosin filaments at different ATP concentration, using a new system that we developed in our laboratory that allows mechanical measurements and visualization of the filaments simultaneously. We also measured the actomyosin ATPase activity of the filaments to correlate with force. Methods: Two types of myosin filaments were used in this study; smooth myosin filaments were isolated from the ABRM of M. edulis, and skeletal myosin filaments were isolated from rabbit psoas muscles. An experimental chamber was mounted on an inverted microscope with fluorescence imaging. The force produced by the myosin filaments upon interaction with fluorescence actin at four different concentration of ATP, 0.5, 0.75, 1.0 and 1.25mM was calculated using the displacement of the cantilevers of known stiffness. (Kalganov et al., 2013). Mg-ATPase activity was measured using methods previously described (Shelud'ko, et al., 2007). Results and Discussion: Forces in both types of myosin filaments were increased consistently with increasing ATP concentration from 0.5mM to 1.25mM during actomyosin interactions. The force produced by smooth and skeletal muscle myosin filaments were 67.05pN/mm and 54.62pN/mm at 0.5mM ATP, and 103.27pN/mm and 112.51pN/mm at 1.25mM ATP, respectively. The force generated by smooth and skeletal muscle myosin filaments were increased approximately 54.0% and 100% when ATP concentration increased from 0.5mM to 1.25mM. ATPase activity of smooth and skeletal myosin was significant different (p<0.05), suggesting that higher ATP concentration will be hydrolyzed to enhance the mechanical work during actomyosin interactions.

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