Distribution of the intermediate elements operating in ER to Golgi transport

We have used a 58 kDa membrane protein (p58) as a marker to study the transport pathway between the rough endoplasmic reticulum (ER) and the Golgi apparatus. Immunolocalization of p58 in fibroblasts showed its presence in a single cisterna and in small tubular and vesicular elements at the cis side of the Golgi apparatus. In addition, the protein was detected in large (200–500 nm in diameter) tubulovesicular structures, clustered in the Golgi region but also found in peripheral locations. These represent intermediates in ER to Golgi transport since they contained newly synthesized viral glycoproteins, arrested in cells at I5°C. The peripheral structures accumulated at low temperature but reclustered rapidly to the Golgi region upon shift of cells back to 37°C. This movement involved long intracellular distances and was efficiently inhibited by nocodazole, indicating that it requires the integrity of microtubules. In contrast, reclustering was unaffected by brefeldin A (BFA), suggesting that this compound affects ER to Golgi transport prior to the temperature-sensitive step. In BFA-treated cells p58 was localized to scattered, tubular, smooth ER clusters, found in close association with rough ER cisternae. The cellular distribution of the intermediate elements indicates that the sites of protein exit are widely distributed within the rough ER network. We suggest that the smooth ER locations where p58 accumulates in BFA-treated cells could represent such peripheral exit sites.