DNA-BINDING AND NUCLEAR TRANSLOCATION OF INSECT HIGH-MOBILITY-GROUP PROTEIN-1 (HMG1) PROTEINS ARE INHIBITED BY PHOSPHORYLATION

The cells of the dipteran insects Chironomus and Drosophila contain hi gh-mobility-group proteins (HMG) that are homologous to the HMG1 prote ins of mammals, but contain only one HMG1 box instead of two. The C-te rminal portions of both mammalian and insect HMG1 proteins comprise ma ny charged residues that modulate the DNA-binding affinity of the HMG1 boxes and were found in Chironomus to be substrates for protein kinas e C (PKC) in vitro and in vivo. Phosphorylation of Chironomus HMG1 pro teins cHMG1a and cHMG1b by PKC resulted in a tenfold and fivefold redu ction, respectively, of the DNA-binding strength. Phosphorylated and u nphosphorylated cHMG1a protein was labelled with fluoresceine isothioc yanate and microinjected into the cytoplasm of Chironomus salivary gla nd cells. The translocation of phosphorylated cHMG1a into the nuclei w as found to be remarkably delayed as compared to that of the unmodifie d form. The distribution of HMG1 proteins between nucleus and cytoplas m is known to vary according to the cell type and the state of differe ntiation. Our results suggest that this distribution may be regulated by changing the efficiency of nuclear translocation and the affinity f or DNA via phosphorylation and dephosphorylation.