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-binding domains (DBD), properties of which appear to be regulated by phosphorylation. High levels of these proteins are characteristic for rapidly dividing cells in embryonic tissues and tumors. On the basis of their occurrence, specific functions for each of these proteins have been postulated. In this study we demonstrate differences in the nature of contacts of these proteins with promoter region of the interferon-β gene. We show that HMGI and HMGY interact with this DNA via three DBDs, whereas HMGI-C and Chironomus HMGI bind to this DNA using only two domains. Phosphorylation of HMGY protein by Cdc2 kinase leads to impairing of contacts between the N-terminally located DBD and a single promoter element. The perturbations in the architecture of the protein·DNA complexes involve changes in the degree of unbending of the intrinsically bent IFNβ promoter. Our results provide first insights into the molecular basis of functional specificity of proteins of the HMGI/Y family and their regulation by phosphorylation.