Mobile ER-to-Golgi but not post-Golgi membrane transport carriers disappear during the terminal myogenic differentiation.

The organelles of the exocytic pathway undergo a profound reorganization during the myogenic differentiation. Here, we have investigated the dynamics of the membrane trafficking at various stages of the differentiation process by using the green fluorescent protein-tagged, temperature-sensitive vesicular stomatitis virus G protein (tsG-GFP) as a marker. At the restrictive temperature of 39°C, the tsG-GFP located to the endoplasmic reticulum (ER) at each stage of differentiation. Mobile membrane containers moving from the ER to the Golgi elements were seen in myoblasts and myotubes upon shifting the temperature to 20°C. In adult myofibers, in contrast, such containers were not seen although the tsG-GFP rapidly shifted from the ER to the Golgi elements. The mobility of tsG-GFP in the myofiber ER was restricted, suggesting localization in an ER sub-compartment. Contrasting with the ER-to-Golgi trafficking, transport from the Golgi elements to the plasma membrane involved mobile transport containers in all differentiation stages. These findings indicate that ER-to-Golgi trafficking in adult skeletal myofibers does not involve long-distance moving membrane carriers as occurs in other mammalian cell types.