Obscurins are cytoskeletal proteins with structural and regulatory roles encoded by OBSCN. Mutations in OBSCN<br>are associated with the development of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy<br>(DCM). Specifically, the R4344Q mutation present in immunoglobulin domain 58 (Ig58) was the first to be linked<br>with the development of HCM. To assess the effects of R4344Q in vivo, we generated the respective knock-in<br>mouse model. Mutant obscurins are expressed and incorporated normally into sarcomeres. The expression<br>patterns of sarcomeric and Ca2+-cycling proteins are unaltered in sedentary 1-year-old knock-in myocardia, with<br>the exception of sarco/endoplasmic reticulum Ca2+ adenosine triphosphatase 2 (SERCA2) and pentameric phospholamban<br>whose levels are significantly increased and decreased, respectively. Isolated cardiomyocytes from<br>1-year-old knock-in hearts exhibit increased Ca2+-transients and Ca2+-load in the sarcoplasmic reticulum and<br>faster contractility kinetics. Moreover, sedentary 1-year-old knock-in animals develop tachycardia accompanied<br>by premature ventricular contractions, whereas 2-month-old knock-in animals subjected to pressure<br>overload develop a DCM-like phenotype. Structural analysis revealed that the R4344Q mutation alters the<br>distribution of electrostatic charges over the Ig58 surface, thus interfering with its binding capabilities.<br>Consistent with this, wild-type Ig58 interacts with phospholamban modestly, and this interaction is markedly<br>enhanced in the presence of R4344Q. Together, our studies demonstrate that under sedentary conditions, the<br>R4344Q mutation results in Ca2+ deregulation and spontaneous arrhythmia, whereas in the presence of<br>chronic, pathological stress, it leads to cardiac remodeling and dilation. We postulate that enhanced binding<br>between mutant obscurins and phospholamban leads to SERCA2 disinhibition, which may underlie the observed<br>pathological alterations.