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The U1A protein is a sequence-specific RNA binding protein found in the U1 snRNP particle where it binds to stem/loop II of U1 snRNA. U1A contains two 'RNP' or 'RRM' (RNA Recognition Motif) domains, which are common to many RNA-binding proteins. The N-terminal RRM has been shown to bind specifically to the U1 RNA stem/loop, while the RNA target of the C-terminal domain is unknown. Here, we describe experiments using a 102 amino acid N-terminal RRM of U1A (102A) and a 25-nucleotide RNA stem/loop to measure the binding constants and thermodynamic parameters of this RNA:protein complex. Using nitrocellulose filter binding, we measure a dissociation constant KD = 2 x 10(-11) M in 250 mM NaCl, 2 mM MgC2, and 10 mM sodium cacodylate, pH 6 at room temperature, and a half-life for the complex of 5 minutes. The free energy of association (delta G degrees) of this complex is about -14 kcal/mol in these conditions. Determination of the salt dependence of the binding suggests that at least 8 ion-pairs are formed upon complex formation. A mutation in the RNA loop sequence reduces the affinity 10 x, or about 10% of the total free energy.

Familial dilated cardiomyopathy (DCM) is a leading cause of sudden cardiac death and a major indicator for heart transplant. The disease is frequently caused by mutations of sarcomeric proteins; however, it is not well understood how these molecular mutations lead to alterations in cellular organization and contractility. To address this critical gap in our knowledge, we studied the molecular and cellular consequences of a DCM mutation in troponin-T, ΔK210. We determined the molecular mechanism of ΔK210 and used computational modeling to predict that the mutation should reduce the force per sarcomere. In mutant cardiomyocytes, we found that ΔK210 not only reduces contractility but also causes cellular hypertrophy and impairs cardiomyocytes’ ability to adapt to changes in substrate stiffness (e.g., heart tissue fibrosis that occurs with aging and disease). These results help link the molecular and cellular phenotypes and implicate alterations in mechanosensing as an important factor in the development of DCM.

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27Familial dilated cardiomyopathy (DCM) is a leading cause of sudden cardiac death and a 28 major indicator for heart transplant. The disease is frequently caused by mutations of 29 sarcomeric proteins; however, it is not well understood how these molecular mutations 30 lead to alterations in cellular organization and contractility. To address this critical gap in 31 our knowledge, we studied the molecular and cellular consequences of a DCM mutation 32 in troponin-T, DK210. We determined the molecular mechanism of DK210 and used 33 computational modeling to predict that the mutation should reduce the force per 34 sarcomere. In mutant cardiomyocytes, we found that DK210 not only reduces contractility, 35 but also causes cellular hypertrophy and impairs cardiomyocytes' ability to adapt to 36 changes in substrate stiffness (e.g., heart tissue fibrosis that occurs with aging and 37 disease). These results link the molecular and cellular phenotypes and implicate 38 alterations in mechanosensing as an important factor in the development of DCM. 39Results 100 DK210 decreases calcium sensitivity in an in vitro motility assay 101We set out to decipher the molecular mechanism of the DK210 mutation in vitro. 102The molecular effects of cardiomyopathy mutations depend on the myosin isoform (7-9, 103 35-37) and therefore, we used porcine cardiac ventricular myosin (38). Porcine ventricular 104 cardiac myosin (MYH7) is 97% identical to human, while murine cardiac myosin (MYH6) 105 is only 92% identical. Porcine cardiac myosin has very similar biophysical properties to 106 human cardiac myosin, including the kinetics of the ATPase cycle, step size, and 107 sensitivity to load (38-41), making it an ideal myosin for biophysical studies. 108Given the role of troponin-T in thin filament regulation, we first determined whether 109 the DK210 mutation affects calcium-based regulation of myosin binding to thin filaments 110 using an in vitro motility assay (42). Reconstituted thin filaments, consisting of porcine 111 cardiac actin and recombinantly expressed human troponin and tropomyosin, were added 112 to a flow cell coated with porcine cardiac myosin in the presence of ATP. The speed of 113 filament translocation was measured as a function of added calcium. As has been 114 reported previously, the speed of regulated thin filament translocation increased 115 sigmoidally with increasing Ca 2+ concentration (43), ( Figure 1B). Data were fit with the Hill 116 equation to obtain the pCa50 (i.e., the concentration of calcium necessary for half-117 maximal activation). Consistent with previous studies using mouse cardiac, rabbit cardiac, 118 and rabbit skeletal muscle fibers (31, 33, 44), DK210 shows a right-shifted curve (pCa50 119 = 5.7 ± 0.1) compared to the WT (pCa50 = 6.1 ± 0.1; p < 0.0001), meaning more calcium 120 is needed for the same level of activation. This suggests that the mutant could show 121 impaired force production during a calcium transient. 122 7 123 Molecular mechanism of DK210-induced changes in thin filament regulat...