CDC2, AND MITOGEN-ACTIVATED PROTEIN-KINASES MODULATE DNA-BINDING PROPERTIES OF THE PUTATIVE TRANSCRIPTIONAL REGULATOR CHIRONOMUS HIGH-MOBILITY GROUP PROTEIN-I

Cells of the dipteran insect Chironomus contain a high mobility group protein that is homologous to the mammalian high mobility group protei ns I/Y (HMGI/Y). These proteins facilitate the assembly of higher orde r nucleoprotein complexes, In proliferating cells, >30% of Chironomus HMGI was found to be phosphorylated, The phosphorylation sites were ma pped to Ser(3), Ser(22), and Ser(72) and were found to be substrates f or the kinases Cdc2 (and mitogen-activated protein (MAP)), MAP, and Ca 2+/phospholipid-dependent protein kinase, respectively, In mitotically arrested cells, the extent of phosphorylation at Ser(3) increased, wh ereas phosphorylation at Ser(22) remained unchanged, In nondividing ce lls, phosphorylation at Ser(3) and Ser(22) was strongly reduced, The D NA binding affinity of Chironomus HMGI was not influenced by single ph osphorylation at Sers or Ser(22). In contrast, phosphorylation at both of these sites resulted in a 10-fold weakening of the binding activit y and altered the mode of protein-DNA interaction. Since both human an d murine HMGI/Y proteins, similarly to the insect HMGI protein, posses s phosphorylation sites for Cdc2 and MAP kinases that intersperse the AT-hook DNA-binding motifs, our results may reflect a general mechanis m that regulates the properties and function of this class of putative transcriptional regulators.