The Intraluminal Domain of Junctin Contains Junctional Sarcoplasmic Reticulum Targeting Sequences

Abstract

Junctin is an integral membrane protein of the sarcoplasmic reticulum (SR) encoded by the Aspartyl beta-hydroxylase (AβH) gene. The AβH gene can undergo different alternative splicing events, generating at least three distinct proteins, the Aspartyl beta-hydroxylase, junctin and junctate, all sharing the same transmembrane domain and a short highly charged acidic luminal region, but carrying distinct intraluminal tails. Interstingly, junctin and junctate are both expressed in skeletal muscle, but only junctin was shown to interact and co-localize with the Ca2+ release complex in the junctional SR, whereas junctate displays a typical longitudinal SR distribution. These two proteins may thus represent an ideal molecular model to investigate the presence of specific regions responsible for protein targeting to the junctional SR.To this aim, junctin deletion mutants were tagged with a green fluorescent protein (GFP) and expressed in primary myoblasts. The results obtained showed that even deletion of small regions within intraluminal domain of junctin resulted in protein mislocalization, thus indicating that the entire luminal sequence is required for protein targeting to the junctional SR. Interestingly, the same region, if inserted downstream the cytoplasmic and trasmembrane domains of sarcolipin, a resident protein of the longitudinal SR, is able to completely re-localize the protein to the junctional SR, further confirming that the intraluminal domain of junctin may actually contain minimal junctional SR targeting sequences.