We have used a novel technique to study the synthesis, modification and degradation of proteins during the cell cycle in Saccharomyces cerevisiae. Logarithmically growing cells were pulse-labeled twice, with the pulses separated in time by more than one generation. Subsequently, the cells were fractionated as to their position in the cell cycle by centrifugal elutriation, and for different proteins the ratio of radioactive material from the two pulses was then determined. Periodic degradation, synthesis, or modification would produce periodic variations in the ratio of counts. Two-dimensional gel electrophoresis was used to examine 110 different proteins at different times of the cell cycle. All but two proteins had a constant ratio of counts through the cell cycle. This indicates that the rate of synthesis of individual proteins increases exponentially during the cell cycle and that periodic degradation or modification of proteins is not a general feature of the cell cycle in S. cerevisiae.
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