Distinct organization of DNA complexes of various HMGI/Y family proteins and their modulation upon mitotic phosphorylation

High mobility group (HMG) proteins HMGI, HMGY, HMGI-C, and Chironomus HMGI are DNA-binding proteins thought to modulate the assembly and the function of transcriptional complexes. Each of these proteins contains three DNA-bin ding domains (DBD), properties of which appear to be regulated by phosphory lation. High levels of these proteins are characteristic for rapidly dividi ng cells in embryonic tissues and tumors. On the basis of their occurrence, specific functions for each of these proteins have been postulated. In thi s study we demonstrate differences in the nature of contacts of these prote ins with promoter region of the interferon-p gene. We show that HMGI and HM GY interact with this DNA via three DBDs, whereas HMGI-C and Chironomus HMG I bind to this DNA using only two domains. Phosphorylation of HMGY protein by Cdc2 kinase leads to impairing of contacts between the N-terminally loca ted DBD and a single promoter element. The perturbations in the architectur e of the protein DNA complexes involve changes in the degree of unbending o f the intrinsically bent TFN beta promoter. Our results provide first insig hts into the molecular basis of functional specificity of proteins of the H MGI/Y family and their regulation by phosphorylation.