Cell cycle changes and the ability of cells to undergo virus-induced fusion

Synchronized cells of the mouse leukemic cell line, L5178Y, were examined for their ability to undergo virus-induced fusion. Fusion capacity (defined as the percentage of the original unicellular population that is induced by virus to participate in a fusion event) was measured in cells that were (i) arrested by colcemid, (ii) released from colcemid block, and (iii) released from thymidine block. The results show that fusion capacity is minimal during G1 phase (less than 10%), and rises to a maximum (69%) in arrested mitosis. Treatment of nonsynchronized cells with trypsin increased their fusion capacity to the level characteristic of cells arrested in mitosis. Colcemid-arrested cells produced twice as many hybrid clones as nonsynchronized cells after they were plated in selective medium.