Effect of dithiothreitol on plasma membrane intramembranous particle topography in BALB/c 3T3 and simian virus-transformed 3T3 cells and plasma membrane vesicles.

A statistical analysis of plasma membrane intramembranous particle topography was performed on purified 3T3 and simian virus (SV)-transformed 3T3 plasma membrane vesicles and on intact 3T3 and SV3T3 cells. The results show that intramembranous particles were more aggregated in intact 3T3 cells than in intact SV3T3 cells. By contrast, in purified plasma membrane vesicles intramembranous particles were significantly more aggregated in SV3T3 than in 3T3 preparations. These unexpected results suggest that intramembranous particle topography is selectively altered in SV3T3 plasma membranes during the membrane vesicle isolation procedure. Inasmuch as the disulfide-reducing agent dithothreitol (DTT) was used to isolate membranes, tests were done to determine if DTT would preferentially affect particle topography in intact SV3T3 cells. Exposure of intact SV3T3 cells to 10 or 50 mM DTT induced intramembranous particle aggregation but had no effect on intact 3T3 cells. Preliminary studies also showed that other sulfhydrylreactive compounds preferentially induced particle aggregation in SV3T3 specimens. These observations suggest that the state of cross-linking of membrane-associated proteins by disulfide bonding influences intramembranous particle topography and that differences exist in plasma membrane-associated sulfhydryl proteins in nontransformed and transformed cells.