Plasma membrane intramembranous particle topography in 3T3 and SV3T3 cells: the effect of cytochalasin B

The topography of plasma membrane intramembranous particles in contact-inhibited and transformed cells is a topic of current controversy. We have reported that particles are aggregated in 3T3 cells and that they are randomly distributed in SV3T3 cells. Other investigators have, however, proposed that these differences result from the effect of cryoprotectants and are blocked by aldehyde fixation. The experiments described in this paper re-examine PMP topography in large samples of 3T3 and SV3T3 cells prepared by a variety of preparative methods. The results confirm that 3T3 cells contain aggregated intramembranous particles and that native SV3T3 cells do not, regardless of whether cells are prepared in glycerol, sucrose, tissue culture medium or following prefixation in paraformaldehyde. Data presented in this paper also show that cytochalasin B induces dose- and time-dependent disaggregations of particles in contact-inhibited 3T3 cells. Particle disaggregation results from exposure of cells in situ for 1-3 h to 1-500 ng/ml cytochalasin B. Removal of the drug by washing with reincubation of cells in medium containing serum results in particle reaggregation within 1 to 3 h. These observations suggest that intramembranous particles in 3T3 cells are aggregated in the native state and that membrane-active drugs modulate their distribution by a mechanism possibly involving disruption of linkages between membrane components and the cytoskeleton.