Synthesis of unique low molecular weight proteins during late G2 and mitosis

Changes in protein synthesis were examined during the cell cycle of Chinese hamster ovary cells by labeling synchronized cells at various times with [35S]methionine and separating the proteins on two-dimensional polyacrylamide gels. Several proteins, including tubulin, showed marked differences in their relative rates of synthesis during the cell cycle. A few proteins were found to be synthesized at a specific time during the cycle. In particular, a pair of proteins of approximately 21,000 daltons and isoelectric point of 5.5 were found to be synthesized only in late G2 and mitotic cells. Cells that were labeled during mitosis and then allowed to divide showed no trace of these proteins, indicating that their presence is transient and that they are likely involved in mitosis.