Changes in microtubule phosphorylation during cell cycle of HeLa cells

The phosphorylation in vitro and in vivo of tubulin isolated from HeLa cells has been examined during the cell cylce. The results obtained indicate that: (a) the protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) activity present in the microtubules, and measured in vitro with exogenous casein as substrate, is maximal in M cells, whereas that present in the cytosol is nearly constant during the S, G-2, and M stages, and decreases during G-1; (b) the patterns through the cell cycle of the maximal number of tubulin sites phosphorylated in vitro and of the microtubular protein kinase activity are similar; (c) the degree of tubulin phosphorylation in vivo is 2- to 3-fold higher in the microtubules isolated from the S and M stages of the cell cycle, than those from G-1 and G-2. This variable phosphate content of tubulin through the cell cycle suggests that such covalent modification might be important to enable tubulin to carry over some of its functions during the cell cycle.