Restricted distribution of mRNAs encoding a sarcoplasmic reticulum or transverse tubule protein in skeletal myofibers.

Calsequestrin (CSQ) and dihydropyridine receptor (DHPR) are muscle cell proteins that are directed into the endoplasmic reticulum (ER) during translation. The former is subsequently found in the sarcoplasmic reticulum (SR) and the latter in the transverse tubule membrane. To elucidate the potential role of mRNA targeting within muscle cells, we have analyzed the localization of CSQ and DHPR proteins and mRNAs in primary cultured rat myotubes, in skeletal muscle cryosections, and in isolated flexor digitorum brevis muscle fibers. In the myotube stage of differentiation, the mRNAs distributed throughout the cell, mimicking the distribution of the endogenous ER marker proteins. In the adult skeletal myofibers, however, both CSQ and DHPRalpha1 transcripts located perinuclearly and in cross-striations flanking Z lines beneath the sarcolemma, a distribution pattern that sharply contrasted the interfibrillar distribution of typical ER proteins. Interestingly, all nuclei of the myofibers were transcriptionally active. In summary, the mRNAs encoding either a resident SR protein or a transverse tubule protein were located beneath the sarcolemma, implying that translocation of the respective proteins to the lumen of ER takes place at this location.