Anna Park

Junctophilin-2 (JPH2) is a junctional membrane-binding protein that plays a role in connecting the sarcoplasmic reticulum to the plasma membrane in cardiac cells. While there are four JPH isoforms, JPH2 is the primary isoform in cardiac muscle cells. JPH2 regulates the sarcoplasmic ryanodine receptor calcium (Ca2+) channels (RyR2) and T-tubule formation/structure. The JPH2 protein is composed of several repeating MORN motifs linked by a joining region, followed by an alpha helical and a divergent region. JPH2 mutants are implicated in hypertrophic cardiomyopathy and many of the mutants that cause hypertrophic cardiomyopathy exist in the MORN motifs and joining regions. We showed that JPH2 interacts with mitofusin-2 (MFN2), an outer mitochondrial membrane protein responsible for several mitochondrial activities, and that the N-terminal third of JPH2 is responsible for MFN2 binding. It is possible that altered JPH2 and MFN2 binding could cause mitochondrial dysfunction in hypertrophic cardiomyopathy. In order to further elucidate the role of JPH2 mutants in the pathogenicity of hypertrophic cardiomyopathy, we generated four JPH2 mutations (S101R, Y129D, E169K and A189T) in the N-terminal third of JPH2. We then performed pull down studies to identify and quantify the degree of binding between the JPH2 variants and MFN2. Of the four JPH2 mutations, S101R and Y129D did not affect the binding interaction between JPH2 and MFN2. However, the mutations E169K and A189T impaired the binding interaction between the two proteins. Interestingly, these two mutations are located in the same motif of the JPH2 protein: the joining region. Future studies will examine the potential role of this joining region in the binding interaction of JPH2 and MFN2.