Cell-cycle-related variation in proteins in suspension-cultured rice cells

To understand the cell cycle process in plants, we searched for proteins that quantitatively change during the cell cycle in suspension-cultured rice ( Oryza sativa L.) cells. The proteins were analyzed by a two-dimensional polyacrylamide gel electrophoresis image-analysis system. We detected 11 proteins that quantitatively changed during the cell cycle, among which beta-tubulins and a calreticulin-like protein were identified. The amounts of beta-tubulin proteins were low in the M phase and high in the G1 phase. In contrast, mRNAs for two of the three types of beta-tubulin were high in the M phase of the cell cycle. The addition of protease inhibitors MG132 or E64d to the cells decreased the beta-tubulin proteins during 24 h, suggesting that beta-tubulin proteins are degraded in vivo by proteases other than those whose activities are inhibited by MG132 or E64d.