Molecular cloning of junctin from human and developing rabbit heart

Canine junctin is a 26-kDa transmembrane protein found in the sarcoplasmic reticulum (SR) membrane in cardiac and skeletal muscle. Junctin has recently been shown to bind directly to calsequestrin, the ryanodine receptor, and triadin. Junctin is thought to play a role in facilitating (and perhaps regulating) Ca(2+) release from the SR. Immature heart exhibits decreased utilization of SR Ca(2+) stores for cell contraction. We have cloned human and rabbit cardiac junctin and investigated the expression of junctin in developing rabbit heart. Human junctin was cloned from an adult cardiac cDNA library. Rabbit junctin was cloned by RT-PCR. Northern blot analysis demonstrates a single primary mRNA transcript of approximately 2.8 kb in hearts from both species. Sequence analysis demonstrates greater than 97% homology between the predicted amino acid sequences of human, rabbit, and canine junctin in the putative transmembrane domain and subsequent initial 61 amino acid portion of the putative luminal domain. These domains also exhibit sequence homology with triadin. The C-terminal region shows much lower (72 to 75%) sequence homology among the three species. In addition, Northern blot analysis demonstrates that the expression of junctin increases markedly in postnatal rabbit myocardium. These findings suggest that the putative transmembrane domain and subsequent initial portion of the putative luminal domain of junctin play an important role in the binding of junctin to calsequestrin, the ryanodine receptor, and triadin in the postnatal heart. Furthermore, the previously described increase in SR Ca(2+) release with development is associated with the increased expression of junctin.