Differential chromatin association and nucleosome binding of the maize HMGA, HMGB, and SSRP1 proteins

In plants, chromosomal high mobility group (HMG) proteins have been identif ied in the HMGA family, containing A/T-hook DNA binding motifs, and in the HMGB family, containing an HMG-box DNA binding domain, that are considered architectural factors in chromatin, We have characterized the association o f the HMGA protein, five different HMGB proteins, and the structure-specifi c recognition protein I (SSRP1) with maize chromatin by extraction experime nts using NaCl, ethidium bromide, spermine, and distamycin A. The differenc e in the release of the proteins from chromatin by these reagents indicates that they are differentially associated with chromatin. This was confirmed by treatment of chromatin with micrococcal nuclease, demonstrating that th e HMGA, HMGB2/3. and SSRP1 proteins are enriched in the highly nuclease-sen sitive fraction of chromatin, which is likely to be transcriptionally compe tent. As examined by electrophoretic mobility shift analyses, the HMGA prot ein and the proteins containing an HMG domain (HMGB proteins and SSRP1) bin d specifically to purified maize mononucleosomes that contain a histone oct amer and similar to 165 bp of DNA. The mode of interaction with the nucleos omes differs for HMGA and HMGB proteins. In the case of the HMGB I protein, the full-length protein is required for specific nucleosome binding, as th e individual HMG-box DNA binding domain (which is sufficient for DNA intera ctions) interacts nonspecifically with the nucleosomes. Collectively, these findings indicate that HMGA, the various HMGB proteins, and SSPR1 are diff erentially associated with plant chromatin and may act as architectural fac tors in different nucleoprotein structures.