High‐mobility‐group proteins P1, I and Y as substrates of the M‐phase‐specific p34cdc2/cyclincdc13 kinase

All dividing cells entering the M phase of the cell cycle undergo the transient activation of an M‐phase‐specific histone H1 kinase which was recently shown to be constituted of at least two subunits, p34cdc2 and cyclincdc13. The DNA‐binding high‐mobility‐group (HMG) proteins 1, 2, 14, 17, I, Y and an HMG‐like protein, P1, were investigated as potential substrates of H1 kinase. Among these HMG proteins, P1 and HMG I and Y are excellent substrates of the M‐phase‐specific kinase obtained from both meiotic starfish oocytes and mitotic sea urchin eggs. Anticyclin immunoprecipitates, extracts purified on specific p34cdc2‐binding p13suc1‐Sepharose and affinity purified H1 kinase display strong HMG I, Y and P1 phosphorylating activities, demonstrating that the p34cdc2/cyclincdc13 complex is the active kinase phosphorylating these HMG proteins. HMG I and P1 phosphorylation is competitively inhibited by a peptide mimicking the consensus phosphorylation sequence of H1 kinase. HMG I, Y and P1 all possess the consensus sequence for phosphorylation by the p34cdc2/cyclincdc13 kinase (Ser/Thr‐Pro‐Xaa‐Lys/Arg). HMG I is phosphorylated invivo at M phase on the same sites phosphorylated in vitro by H1 kinase. P1 is phosphorylated by H1 kinase on sites different from the sites of phosphorylation by casein kinase II. The three thermolytic phosphopeptides of P1 phosphorylated in vitro by purified H1 kinase are all present in thermolytic peptide maps of P1 phosphorylated in vivo in proliferating HeLa cells. These phosphopeptides are absent in nonproliferating cells. These results demonstrate that the DNA‐binding proteins HMG I, Y and P1 are natural substrates for the M‐phase‐specific protein kinase. The phosphorylation of these proteins by p34cdc2/cyclincdc13 may represent a crucial event in the intense chromatin condensation occurring as cells transit from the G2 to the M phase of the cell cycle.