Pirani, AlnoorXu, ChenHatch, VictoriaCraig, Roger W.Tobacman, Larry S.Lehman, William2022-08-232022-08-232005-02-172010-10-06J Mol Biol. 2005 Feb 25;346(3):761-72. Epub 2005 Jan 11. <a href="http://dx.doi.org/10.1016/j.jmb.2004.12.013">Link to article on publisher's site</a>0022-2836 (Linking)10.1016/j.jmb.2004.12.01315713461https://hdl.handle.net/20.500.14038/27664The movement of tropomyosin from actin's outer to its inner domain plays a key role in sterically regulating muscle contraction. This movement, from a low Ca2+ to a Ca2+-induced position has been directly demonstrated by electron microscopy and helical reconstruction. Solution studies, however, suggest that tropomyosin oscillates dynamically between these positions at all Ca2+ levels, and that it is the position of this equilibrium that is controlled by Ca2+. Helical reconstruction reveals only the average position of tropomyosin on the filament, and not information on the local dynamics of tropomyosin in any one Ca2+ state. We have therefore used single particle analysis to analyze short filament segments to reveal local variations in tropomyosin behavior. Segments of Ca2+-free and Ca2+ treated thin filaments were sorted by cross-correlation to low and high Ca2+ models of the thin filament. Most segments from each data set produced reconstructions matching those previously obtained by helical reconstruction, showing low and high Ca2+ tropomyosin positions for low and high Ca2+ filaments. However, approximately 20% of segments from Ca2+-free filaments fitted best to the high Ca2+ model, yielding a corresponding high Ca2+ reconstruction. Conversely, approximately 20% of segments from Ca2+-treated filaments fitted best to the low Ca2+ model and produced a low Ca2+ reconstruction. Hence, tropomyosin position on actin is not fixed in either Ca2+ state. These findings provide direct structural evidence for the equilibration of tropomyosin position in both high and low Ca2+ states, and for the concept that Ca2+ controls the position of this equilibrium. This flexibility in the localization of tropomyosin may provide a means of sterically regulating contraction at low energy cost.en-USActinsAnimalsBinding SitesCalciumCattleImage Processing, Computer-AssistedMicroscopy, ElectronModels, MolecularMultiprotein ComplexesMuscle ContractionMuscle ProteinsMuscle RelaxationMuscle, SkeletalMyocardial ContractionMyocardiumRabbitsTropomyosinTroponinCell BiologyJournal Articlehttps://escholarship.umassmed.edu/craig/171594916craig/17