The contribution of cardiac myosin binding protein‐c Ser282 phosphorylation to the rate of force generation and in vivo cardiac contractility
Authors: Gresham Kenneth S. , Mamidi Ranganath , Stelzer Julian E. 
Published: June 2014

Key points
Phosphorylation of cardiac myosin binding protein‐C Ser282 has been proposed to modulate the phosphorylation of Ser273 and Ser302, and thereby the contractile response to increased β‐adrenergic stimulation, yet the precise functional role of Ser282 is unknown.
Protein kinase A phosphorylation of Ser273 and Ser302 was unaffected by Ser282 phospho‐ablation, suggesting that Ser282 phosphorylation is not required for full phosphorylation of neighbouring residues.
Mice with Ser282 phospho‐ablation (TGS282A) displayed normal basal in vivo cardiac function but impaired rates of pressure development in response to β‐adrenergic stimulation.
Basal rates of cross‐bridge kinetics were unaffected by Ser282 phospho‐ablation; however, the protein kinase A‐mediated acceleration of cross‐bridge recruitment was blunted in TGS282A myocardium.
Collectively, our data suggests that Ser282 phosphorylation is critical to achieve complete acceleration of cardiac contractile function in response to increased β‐adrenergic stimulation, but also implicates Ser273 and Ser302 phosphorylation as important modulators of the cardiac myosin binding protein‐C‐mediated contractile response.
AbstractCardiac myosin binding protein‐C phosphorylation plays an important role in modulating cardiac muscle function and accelerating contraction. It has been proposed that Ser282 phosphorylation may serve as a critical molecular switch that regulates the phosphorylation of neighbouring Ser273 and Ser302 residues, and thereby govern myofilament contractile acceleration in response to protein kinase A (PKA). Therefore, to determine the regulatory roles of Ser282 we generated a transgenic (TG) mouse model expressing cardiac myosin binding protein‐C with a non‐phosphorylatable Ser282 (i.e. serine to alanine substitution, TGS282A). Myofibrils isolated from TGS282A hearts displayed robust PKA‐mediated phosphorylation of Ser273 and Ser302, and the increase in phosphorylation was identical to TG wild‐type (TGWT) controls. No signs of pathological cardiac hypertrophy were detected in TGS282A hearts by either histological examination of cardiac sections or echocardiography. Baseline fractional shortening, ejection fraction, isovolumic relaxation time, rate of pressure development and rate of relaxation (τ) were unaltered in TGS282A mice. However, the increase in cardiac contractility as well as the acceleration of pressure development observed in response to β‐adrenergic stimulation was attenuated in TGS282A mice. In agreement with our in vivo data, in vitro force measurements revealed that PKA‐mediated acceleration of cross‐bridge kinetics in TGS282A myocardium was significantly attenuated compared to TGWT myocardium. Taken together, our data suggest that while Ser282 phosphorylation does not regulate the phosphorylation of neighbouring Ser residues and basal cardiac function, full acceleration of cross‐bridge kinetics and left ventricular pressure development cannot be achieved in its absence.