Region of insect high mobility group (HMG) 1 protein homologous to helix 2 of the rat HMG1-b box is in close contact with DNA.

The dipteran insects Chironomus and Drosophila have high mobility group (HMG) 1 proteins that are similar to mammalian HMG1 but contain one instead of two HMG1 boxes. The interaction of the Chironomus HMG1 proteins cHMG1a and cHMG1b with double-stranded and four-way junction DNA was analyzed by investigating the accessibility of defined sequences of the HMG1 box to specific antibodies within the DNA-protein complex in vitro. Antibody epitopes located in the three helices and in the turn between helices 1 and 2 of the HMG1 box were identified on a membrane onto which 90 decapeptides with overlapping sequences spanning the entire sequence of cHMG1a had been bound. Monospecific antibodies directed against selected epitopes were purified from a polyclonal antiserum by affinity chromatography. Helices 1 and 3 as well as the turn between helices 1 and 2 were found to be accessible to antibodies in the complex. In contrast, antibodies recognizing an epitope on putative helix 2 of cHMG1a and cHMG1b were unable to produce supershifts on gels of the DNA-protein complexes with both DNAs. These data suggest that helix 2 of the HMG1 box of proteins cHMG1a and cHMG1b is essentially responsible for contacts with DNA.