The role of microtubules in the brefeldin
A (BFA)-associated relocation of major histocompatibility complex (MHC) class II ␣ chains (␣) and the invariant chain (Ii) was characterized in Raji cells by the use of nocodazole (ND). BFA blocked the transport of ␣BIi proteins through the Golgi and redistributed them to the endoplasmic reticulum (ER) along with Golgi-resident enzymes. The result of the colocalization of processing enzymes and newly synthesized proteins was a downshift of ␣Ii molecular weight (MW) of 2 kDa, and their resistance to endoglycosidase H (endo H) after 6 hr of chase. ND by itself had no effect on the processing and transport of ␣ to the cell surface. The addition of ND to BFA-treated cells downshifted ␣Ii by 4 kDa. Additionally, ␣Ii proteins remained sensitive to neuraminidase after 16 hr of chase. In vitro ␣-mannosidase treatment of immunoprecipitated ␣Ii generated a similar 4-kDa downshift of MW. Either 1-deoxymannojirimycin (DJN) or swainsonine (SWN) blocked the MW downshift caused by BFA + ND treatment. These observations indicated that in Raji cells, most of the BFA-associated relocations of cis-, medial Golgi proteins, and the addition of sialic acid from the trans-Golgi were microtubule-independent. The retrograde transport of the medial Golgi enzyme N-acetylglucosamine transferase, however, required microtubular function. Microtubule disrupters could affect BFA treatment of viral infections by further disrupting viral protein processing. Am.