The projection of knobby protuberances at the cell surface (zeiosis) is a general cellular response to cytochalasin D (CD), resulting from herniation of endoplasm through undefended places of the cortex during cell contractions and displacement of microfilaments induced by CD. Zeiosis is prevented by agents that interfere with the contractile response to CD, such as inhibitors of energy metabolism or cyclic AMP. The developed protrusions, which remain relatively stable in the presence of CD, contain chiefly mono- or subribosomes, and occasionally other organelles normally resident in endoplasm; compact microfilament felt occupies their bases and extends into their proximal stalks. Protein synthesis in the knobs is less than half of that in the polyribosome-containing endoplasm residual in the main body of the cell. Knobs first protrude singly near the margin of the contracting cells and rapidly cluster into small groups in the periphery even at lower temperature. The clusters then migrate centripetally and coalesce into a large aggregate near the apex of the immobilized and retracted cell: this movement is energy- and temperature-dependent. Aggregation is more prominent and stable in cell lines of epithelial derivation than in fibroblastic or other lines in which nuclear extrusion occurs more readily. The latter is regarded as a special manifestation of zeiosis. Macromarkers, such as latex spherules, migrate like the zeiotic knobs on the cell surfaces in the presence of CD. The aggregated knobs, although persistent for days in the presence of CD, are rapidly recessed after withdrawal of the agent as ruffling is resumed and the cells spread. These movements are discussed in terms of current concepts of mobility of the cell membrane.
The effect of divalent cations on cell fusion by concentrated Sendai virus, inactivated by beta-propiolactone, was investigated using Vero and mouse L-929 cells in monolayers. With both cell lines, which are normally resistant to exogenous viral fusion, Cu2+ in sublethal concentrations was found to promote polykaryon formation to a marked degree. The simultaneous presence of Cu2+ and virus was required for this effect, which was thought to be related to the cytotoxic action of Cu2+ on the cell membrane. Accordingly, under standard conditions and in the absence of virus, leakage of isotopically labeled intracellular protein was shown to bear a quantitative relationship to Cu2+ concentration. Concomitant changes in the membrane were seen electron microscopically to consist of loss of microvilli and the appearance of numerous vesicles on, or adjacent to, the membrane. The relationship of enhanced fusibility to these toxic changes was not further elucidated. The fusion-promoting effect of Cu2+ far exceeded that of Ca2+; and other cations tested had no effect.