Constitutive phosphorylation of the acidic tails of the high mobility group 1 proteins by casein kinase II alters their conformation, stability, and DNA binding specificity

The high mobility group (HMG) 1 and 2 proteins are the most abundant non-hi stone components of chromosomes. Here, we report that essentially the entir e pool of HMG1 proteins in Drosophila embryos and Chironomus cultured cells is phosphorylated at multiple serine residues located within acidic tails of these proteins. The phosphorylation sites match the consensus phosphoryl ation site of casein kinase II. Electrospray ionization mass spectroscopic analyses revealed that Drosophila HMGD and Chironomus HBG1a and HMG1b are d ouble-phosphorylated and that Drosophila HMGZ is triple-phosphorylated. The importance of this post-translational modification was studied by comparin g some properties of the native and in vitro dephosphorylated proteins. It was found that dephosphorylation affects the conformation of the proteins a nd decreases their conformational and metabolic stability. Moreover, it wea kens binding of the proteins to four-way junction DNA by 2 orders of magnit ude, whereas the strength of binding to linear DNA remains unchanged. Based on these observations, we propose that the detected phosphorylation is imp ortant for the proper function and turnover rates of these proteins. As the occurrence of acidic tails containing canonical casein kinase II phosphory lation sites is common to diverse HMG and other chromosomal proteins, our r esults are probably of general significance.