Brefeldin A-induced alterations in processing of MHC class II-Ii complex depend upon microtubular function

The role of microtubules in the brefeldin A (BFA)-associated relocation of major histocompatibility complex (MHC) class II alphabeta chains (alphabeta) and the invariant chain (Ii) was characterized in Raji cells by the use of nocodazole (ND). BFA blocked the transport of alphabeta Ii 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 alphabeta 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 alphabeta to the cell surface. The addition of ND to BFA-treated cells downshifted alphabeta Ii by 4 kDa. Additionally, alphabeta Ii proteins remained sensitive to neuraminidase after 16 hr of chase. In vitro alpha-mannosidase treatment of immunoprecipitated alphabeta 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.