Closing a small gap in our understanding of excitation-contraction coupling: JSR90 JFP90 is Junctophilin 1

Development of a panel of monoclonal antibodies against triad junctional membranes provided a powerful tool for the characterisation of multiple proteins participating in excitation-contraction coupling (ECC), including the ryanodine receptor (RyR) calcium channel, dihydropyridine receptor voltage-sensor and linker protein triadin. Another component of ECC, termed the 90 kDa junctional foot protein (JFP90 JSR90) was identified with an antibody clone, mAb VF1c. This protein is predominantly expressed in skeletal muscle, is most abundant in fast-twitch fibres and is phosphorylated by an endogenous kinase. Additional studies indicated that it forms a supramolecular complex with the RyR and is upregulated in muscles from aged humans. Despite its potential importance, the molecular identity of JFP90 was not determined at the time. In the current study, the identity of JFP90 JSR90 as deduced by immunoprecipitation with mAb VF1c and proteomic analysis of the major proteins isolated. Using this approach, it was determined that JFP90 is junctophilin 1, a triad-enriched protein that links the sarcoplasmic reticulum to the t-tubules, thereby contributing to ECC. Consolidated with data from previous publications, the current study reveals novel properties of junctophilin 1, in particular that it is selectively upregulated in the soleus muscle, but not quadriceps, of aged mice.