The Arabidopsis Genome Encodes Structurally and Functionally Diverse HMGB-type Proteins

Abstract

The high mobility group (HMG) proteins of the HMGB family are chromatin-associated proteins that act as architectural factors in nucleoprotein structures, which regulate DNA-dependent processes including transcription and recombination. In addition to the previously identified HMGB1-HMGB6 proteins, the Arabidopsis genome encodes at least two other candidate family members (encoded by the loci At2g34450 and At5g23405) having the typical overall structure of a central domain displaying sequence similarity to HMG-box DNA binding domains, which is flanked by basic N-terminal and acidic C-terminal regions. Subcellular localisation experiments demonstrate that the At2g34450 protein is a nuclear protein, whereas the At5g23405 protein is found mainly in the cytoplasm. In line with this finding, At5g23405 displays specific interaction with the nuclear export receptor AtXPO1a. According to CD measurements, the HMG-box domains of both proteins have an alpha-helical structure. The HMG-box domain of At2g34450 interacts with linear DNA and binds structure-specifically to DNA minicircles, whereas the HMG-box domain of At5g23405 does not interact with DNA at all. In ligation experiments with short DNA fragments, the At2g34450 HMG-box domain can facilitate the formation of linear oligomers, but it does not promote the formation of DNA minicircles. Therefore, the At2g34450 protein shares several features with HMGB proteins, whereas the At5g23405 protein has different characteristics. Despite the presence of a region with similarity to the nucleosome-binding domain typical of HMGN proteins, At2g34450 does not bind nucleosome particles. In summary, our data demonstrate (i) that plant HMGB-type proteins are functionally variable and (ii) that it is difficult to predict HMG-box function solely based on sequence similarity.