Identification of a labile protein involved in the G1-to-S transition in Saccharomyces cerevisiae.

The biochemical nature of the start process that commits budding yeast to DNA synthesis is not known. Kinetic evidence has suggested recently that short-lived protein(s) may have to accumulate to a critical level before the cell cycle may progress towards DNA synthesis and cell division. We investigated by high-resolution two-dimensional electrophoresis whether, in a cdc25-1 mutant strain of Saccharomyces cerevisiae that had been blocked at the regulatory step called "start" by growth at a restrictive temperature, short-lived proteins are synthesized during the recovery of growth at a permissive temperature. Of the approximately equal to 500 proteins resolved by the two-dimensional electrophoresis, 6 were short-lived. Only one of them (Mr = 100,000, pI approximately equal to 4.8-5) appears to be specifically made during the G1-to-S transition at start. A regulatory role for cell cycle progression in yeast is suggested for this protein, p100.