In cells treated with brefeldin A (BFA), movement of newly synthesized membrane proteins from the endoplasmic reticulum (ER) to the Golgi apparatus was blocked. Surprisingly, the glycoproteins retained in the ER were rapidly processed by cis/medial Golgi enzymes but not by trans Golgi enzymes. An explanation for these observations was provided from morphological studies at both the light and electron microscopic levels using markers for the cis/medial and trans Golgi. They revealed a rapid and dramatic redistribution to the ER of components of the cis/medial but not the trans Golgi in response to treatment with BFA. Upon removal of BFA, the morphology of the Golgi apparatus was rapidly reestablished and proteins normally transported out of the ER were efficiently and rapidly sorted to their final destinations. These results suggest that BFA disrupts a dynamic membrane-recycling pathway between the ER and cis/medial Golgi, effectively blocking membrane transport out of but not back to the ER.

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