Human actin mutations associated with hypertrophic and dilated cardiomyopathies demonstrate distinct thin filament regulatory properties in vitro

Department of Molecular Physiology & Biophysics, University of Vermont, College of Medicine, 149 Beaumont Drive, Burlington, VT 05405, USA.
Journal of Molecular and Cellular Cardiology (Impact Factor: 5.22). 09/2009; 48(2):286-92. DOI: 10.1016/j.yjmcc.2009.09.014
Source: PubMed

ABSTRACT Two cardiomyopathic mutations were expressed in human cardiac actin, using a Baculovirus/insect cell system; E99K is associated with hypertrophic cardiomyopathy whereas R312H is associated with dilated cardiomyopathy. The hypothesis that the divergent phenotypes of these two cardiomyopathies are associated with fundamental differences in the molecular mechanics and thin filament regulation of the underlying actin mutation was tested using the in vitro motility and laser trap assays. In the presence of troponin (Tn) and tropomyosin (Tm), beta-cardiac myosin moved both E99K and R312H thin filaments at significantly (p<0.05) slower velocities than wild type (WT) at maximal Ca(++). At submaximal Ca(++), R312H thin filaments demonstrated significantly increased Ca(++) sensitivity (pCa(50)) when compared to WT. Velocity as a function of ATP concentration revealed similar ATP binding rates but slowed ADP release rates for the two actin mutants compared to WT. Single molecule laser trap experiments performed using both unregulated (i.e. actin) and regulated thin filaments in the absence of Ca(++) revealed that neither actin mutation significantly affected the myosin's unitary step size (d) or duration of strong actin binding (t(on)) at 20 microM ATP. However, the frequency of individual strong-binding events in the presence of Tn and Tm, was significantly lower for E99K than WT at comparable myosin surface concentrations. The cooperativity of a second myosin head binding to the thin filament was also impaired by E99K. In conclusion, E99K inhibits the activation of the thin filament by myosin strong-binding whereas R312H demonstrates enhanced calcium activation.

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    • "Interestingly, as discussed previously, the E99A/ E100A double mutant was identified in the Wertman alanine scanning mutagenesis screen in yeast where the two together produced a deleterious phenotype [Wertman et al., 1992; Drubin et al., 1993; Miller et al., 1996]. Both the E99K and R312H dilated cardiomyopathy mutations in baculovirus-expressed cardiac actin were also studied [Bookwalter and Trybus, 2006; Debold et al., 2010] for their effects on tropomyosin and troponin regulation. Both mutations resulted in decreased filament motility rates compared with WT actin, and both mutations affected calcium regulation of thin filament activity both at sub-saturating and saturating calcium concentrations. "
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    • "While myosin is detached (or more precisely weakly bound) from actin, ATP is hydrolyzed, a biochemical event coupled to resetting of the lever arm, ensuring that the next binding event causes another productive displacement. It is important to point out that more complex models have been proposed based on recent in vitro findings including additional AM.ADP states that could be coupled to a structural change in the position of the lever arm (Capitanio et al., 2006) and strongly strongly bound pre-powerstroke states (Takagi et al., 2004) but this simple model is consistent with much of the data from in vitro experiments (Palmiter et al., 1999; Baker et al., 2002; Debold et al., 2008, 2010) and provides the best starting point for understanding the effects of elevated levels of Pi, H+, and ADP on actomyosin. In fact, years of investigations using skinned muscle fibers have led to several hypotheses regarding how elevated levels of Pi, H+ and ADP during fatigue could directly inhibit specific steps in the cross-bridge cycle (Cooke, 2007). "
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