High affinity interaction of dipteran high mobility group (HMG) proteins 1 with DNA is modulated by COOH-terminal regions flanking the HMG box domain.

Abstract

The cells of the dipteran insects Chironomus and Drosophila contain high mobility group (HMG) 1 proteins that are homologous to the HMG1 protein of mammals but comprise one instead of two DNA-binding HMG boxes. Mobility shift assays have revealed that Chironomus cHMG1a and cHMG1b bind double strand and four-way junction DNA in a similar way at apparent dissociation constants in the range of 7.5-20 x 10(-9) M. Both proteins are monomeric and highly asymmetric molecules in solution. cHMG1a and cHMG1b exhibit Stokes' radii of 2.4 and 2.3 nm, respectively, and both show a frictional ratio of 1.5. Despite these similarities in their hydrodynamic properties, the binding site of cHMG1a on DNA is approximately 1.5 of the size found for the cHMG1b. Enzymatically and chemically prepared peptides of cHMG1a as well as bacterially expressed cHMG1a with terminal deletions and point substitutions showed that sequences flanking the folded domain that constitutes the HMG box are essential for the interaction of the HMG box with DNA. In particular, changes in the number of positive and negative charges, respectively, within basic and acidic domains modulated the DNA binding affinity of the cHMG1a protein. The alteration of fluorescence of the Trp residues suggest that this modulation is due to interaction of the acidic domain with the positively charged HMG box.