Membrane particle arrays in SV40-transformed 3T3 cells

The ultrastructural organization of the plasma membrane of untransformed and simian virus (SV)40-transformed 3T3 cells in culture was studied by an in situ freeze-fracturing technique. Both 3T3 and SV3T3 cells showed randomly distributed intramembrane particles as well as gap junctions. In addition, the plasma membrane (P-face) of confluent SV40-transformed 3T3 cells was found to display a unique membrane specialization which was not observed in normal 3T3 cells. This membrane specialization consisted of irregularly-shaped arrays of loosely dispersed large (11-12 nm) intramembrane particles, located in regions of close contact between adjacent SV3T3 cells. When a P- to E-fracture face transition occurred within the boundaries of a particle array, the intracellular space appeared uniformly reduced and arrays of pits coextensive with the P-face particles could be recognized on the E-face. Particle arrays appeared particularly prominent in SV3T3 cells grown in the presence of high serum concentrations (20% foetal calf serum or calf serum). Particle arrays resembling those found in SV3T3 cells were observed only in very rare instances in the plasma membrane of other virus-transformed cell lines. Several possible interpretations of the meaning of the particle arrays, including the hypothesis thay they represent an unusual or abortive form of intracellular junction, are discussed. The absence of similar membrane specializations in the plasma membrane of untransformed 3T3 cells points to a difference in membrane organization and cell contact ultrastructure between normal and SV40-transformed 3T3 cells.