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Center for Nonlinear Studies |
Muscles must generate different amounts of force and operate at various speeds to power a wide variety of movements. A major protein component that is critical to setting these mechanical properties is the myosin isoform present in the muscle. For example, the speed at which a myosin isoform moves actin, has been shown to directly correlate with muscle type shortening velocity. However, little is known about how myosin isoform functional variation is determined by myosin structure. Taking advantage of the relatively simple Drosophila myosin expression mechanism, where all muscle myosin isoform are generated from the same gene through alternative splicing, we determined that the converter region, which links the catalytic domain and the lever arm, of myosin is one of the most important for setting functional properties. Interestingly, the converter is also the location of many point mutations that cause familial hypertrophic cardiomyopathy, a leading cause of death in young adults. Specifically, we tested for a possible role of the converter in setting load-dependent myosin properties that vary between muscle fiber types. One manifestation of myosin load dependence is the hyperbolic, rather that linear, shape of the muscle force-velocity relationship (FVR). In Drosophila, there are five endogenous versions of the converter (11a, 11b, 11c, 11d, and 11e) that are expressed in different muscle types. We created five transgenic fly lines each expressing only one of the converters in their jump muscles and mechanically evaluated isolated fiber preparations from these muscles. The converter expressed had a large influence on maximum shortening velocity and FVR curvature but little influence on isometric tension. These changes in the FVR enabled maximum power to be generated at different shortening velocities. To investigate the mechanism behind these changes in FVRs, we constructed five myosin S1 homology models with the various converters. Molecular dynamics simulations found the converter’s flexibility varied with the converter sequence. Specifically, the standard deviation of the distance of the converter to the nearby lever arm alpha helix was variable and correlated strongly with FVR curvature (r2=0.92). In the simulations, converter 11a’s alpha helix displayed the largest range of motion, while 11d’s displayed the least. Analysis of the five structural models revealed distinct differences in amino acid properties and small structural motifs that could be contributing to the differences in converter flexibility. Hydropathicity differentiated the converters and correlated with converter flexibility and FVR curvature (r2=0.85 and 0.83, respectively). We propose that the converter’s flexibility, specifically the mobility of the central alpha helix and lower strand, is at least part of the mechanism behind myosin load dependent properties.
Host: Angel E. Garcia