HMGB6 from Arabidopsis thaliana specifies a novel type of plant chromosomal HMGB protein

The high-mobility group (HMG) proteins of the HMGB family are chromatin-associated proteins that act as architectural factors in various nucleoprotein structures, which regulate DNA-dependent processes such as transcription and recombination. Database analyses revealed that in addition to the previously identified HMGB1-HMGB5 proteins, the Arabidopsis genome encodes at least three other family members having the typical overall structure of a central HMG-box DNA binding domain, which is flanked by basic and acidic regions. These novel HMGB proteins display some structural differences, when compared to HMGB1-HMGB5. Therefore, a representative of the identified proteins, now termed HMGB6, was further analyzed. The HMGB6 protein of similar to27 kDa is the largest plant HMGB protein identified so far. This is essentially due to its unusually extended N-terminal domain of 109 amino acid residues. Subcellular localization experiments demonstrate that it is a nuclear protein. According to CD measurements, HMGB6 has an a-helical HMG-box domain. HMGB6 can bind DNA structure-specifically, and it is a substrate for the protein kinase CK2alpha. Because of these features, HMGB6, and presumably its relatives, can be considered members of the plant HMGB protein family. Hence, eight different chromosomal HMGB proteins are expressed in Arabidopsis, and they may serve specialized architectural functions assisting various DNA-dependent processes.