Changes in membrane structure associated with cell contact

Ultrastructural analysis of 3T3 fibroblasts by freeze cleavage demonstrated significant changes in cell membrane structure associated with cell to cell contact and malignant transformation. These changes consist of a rearrangement and redistribution of intramembranous particles on the membrane fracture faces exposed by freeze cleavage. The results show that noncontacted 3T3 cells in low density contain randomly distributed intramembranous particles. With the development of cell to cell contacts during the logarithmic phase of growth however, a pronounced aggregation of intramembranous particles is seen. A direct correlation between the degree of cell contact and the percentage of cells showing intramembranous particle aggregation has been established. By contrast, transformed simian virus transformed 3T3 and spontaneously transformed 3T3 cells show no evidence of intramembranous particle aggregation even at cell densities where cell to cell contact is extensive. In view of recent reports that intramembranous particles represent foci of interaction between certain intrinsic membrane proteins and lipids, it is proposed that cell to cell contact of nontransformed 3T3 cells may initiate a change in the distribution of intrinsic membrane proteins associated with intramembranous particles and that these changes may influence control of cell proliferation.