M-alpha2/delta promotes myonuclear positioning and association with the sarcoplasmic-reticulum

Abstract

The cytoplasm of striated myofibers contains a large number of membrane organelles, including sarcoplasmic reticulum (SR), T-tubules, and the nuclear membrane. These organelles maintain a characteristic juxtaposition, that appears to be essential for efficient inter-membranous exchange of RNA, proteins, and ions. We found that the membrane associated muscle-specific alpha2/delta subunit of the Ca++ channel complex (Ma2/d) localizes to the SR and T-tubules, and accumulates at the myonuclear surfaces. Furthermore, Ma2/d mutant larval muscles exhibt nuclear positioning defects, disruption of the nuclear-SR juxta positioning, as well as impaired larval locomotion. Ma2/d localization at the nuclear membrane depends on the proper function of the Nesprin orthologue Msp300, and the BAR domain protein Amphiphysin (Amph). Importantly, live imaging of muscle contraction in intact larvae indicated altered distribution of Sarco/Endoplamic Reticulum Ca2+- ATPase (SERCA) around the myonuclei of Ma2/delta mutant larvae. Co-immunoprecipitation analysis supports association between Ma2/d and Amphiphysin, and indirectly with Msp300. We therefore suggest that Ma2/d in association with Msp300 and Amph mediate interactions between the SR and the nuclear membrane.