Cardiomyopathy-causing deletion K210 in cardiac troponin T alters phosphorylation propensity of sarcomeric proteins

Ca²⁺ desensitization of myofilaments is indicated as a primary mechanism for the pathogenesis of familial dilated cardiomyopathy (DCM) associated with the deletion of lysine 210 (ΔK210) in cardiac troponin T (cTnT). ΔK210 knock-in mice closely recapitulate the clinical phenotypes documented in patients with this mutation. Considerable evidence supports the proposition that phosphorylation of cardiac sarcomeric proteins is a key modulator of function and may exacerbate the effect of the deletion. In this study we investigate the impact of K210 deletion on phosphorylation propensity of sarcomeric proteins. Analysis of cardiac myofibrils isolated from ΔK210 hearts identified a decrease in phosphorylation of cTnI (46%), cTnT (30%) and MyBP-C (32%) compared with wild-type controls. Interestingly, immunoblot analyses with phospho-specific antibodies show augmented phosphorylation of cTnT-Thr²⁰³ (28%) and decreased phosphorylation of cTnI-Ser^23/24 (41%) in mutant myocardium. In vitro kinase assays indicate that ΔK210 increases phosphorylation propensity of cTnT-Thr²⁰³ three-fold, without changing cTnI-Ser^23/24 phosphorylation. Molecular modeling of cTnT-ΔK210 structure reveals changes in the electrostatic environment of cTnT helix (residues 203-224) that lead to a more basic environment around Thr²⁰³, which may explain the enhanced PKC-dependent phosphorylation. In addition, yeast two-hybrid assays indicate that cTnT-ΔK210 binds stronger to cTnI compared with cTnT-wt. Collectively, our observations suggest that cardiomyopathy-causing ΔK210 has far-reaching effects influencing cTnI-cTnT binding and posttranslational modifications of key sarcomeric proteins.