Monoclonal antibodies as probes for the complexity, phylogeny, and chromatin distribution of high mobility group chromosomal proteins 1 and 2.

Monoclonal antibodies were prepared against the high mobility group (HMG) proteins 1, 2a, and 2b from hen erythrocyte chromatin. One antibody that recognized multiple sites along HMG-1, -2a, and -2b reacted strongly with HMG proteins from all vertebrates tested. In contrast, five antibodies that detected unique epitopes on chicken HMG-1 and -2a recognized antigenic sites that exhibited restricted phylogenic distributions. The differential reactivity of these antibodies on vertebrate proteins was in agreement with traditional taxonomy in that the avian HMGs were most closely related to those from reptiles and less related to those from mammals, amphibians, bonyfish, and especially the jawless fish. Mononucleosomes generated by mild digestion of erythrocyte chromatin with micrococcal nuclease were highly enriched in HMG-2a. One antigenic determinant located within the N-terminal domain of HMG-2a was freely accessible to its antibody when the protein was bound to these mononucleosomes. In contrast, two antibodies that recognized determinants in the central region of HMG-2a exhibited little chromatin binding activity. The masking of the central domain by DNA binding was presumably not responsible for these results because all three determinants were available for antibody binding when HMG-2a was bound to DNA in vitro. Therefore, the central region of HMG-2a may be masked from antibody binding by protein-protein interactions in chromatin.