Plant chromosomal high mobility group (HMG) proteins

The high mobility group (HMG) proteins were originally discovered in the 1960s as contaminants in calf thymus histone H1 preparations. They were operationally defined as small and abundant, 2% TCA-soluble non-histone proteins extractable from chromatin with 0.35 M NaCl and having a high content of acidic and basic amino acid residues. Therefore, according to this operational definition HMG proteins represent an artificial group of proteins that are presumably functionally unrelated. More recently, based on their primary structure, the HMG proteins of mammalian organisms have been subdivided into three major families. (i) The HMG1/2 family. These proteins (molecular weight 25 000-30 000) have a tripartite structure consisting of two homologous HMG-box DNA-binding domains and a highly acidic carboxy-terminal domain. Although HMG1/2 proteins were initially reported to be single-stranded DNA-binding proteins, they display a high affinity for distorted structures of double-stranded DNA. (ii) The HMG14/17 family. These proteins (molecular weight around 10 000) contain a conserved nucleosome-binding domain which has a higher affinity for nucleosome core particles than for DNA. HMG14/17 proteins are preferentially associated with transcriptionally active chromatin. (iii) The HMGI/Y family. These proteins (molecular weight around 10 000) have three copies of the A/T-hook DNA-binding motif. HMGI/Y proteins bind preferentially to A/T-rich stretches of double-stranded DNA and are more abundant in proliferating cells. The HMG proteins are among the most abundant and ubiquitous non-histone proteins in the nucleus and it appears that at least some HMG proteins (HMG1/2) can shuttle between the nucleus and the cytoplasm. It has been estimated that approximately one in every 10-15 nucleosomes, on average, might be associated with HMG1/2 and HMG14/17. HMG proteins are subject to multiple post-translational modifications such as acetylation, methylation, phosphorylation, ADP-ribosylation and glycosylation, although the significance of these modifications is still unclear. Despite their widespread occurrence, the biological function of these proteins is still largely unknown, although there are numerous reports indicating an involvement in various cellular processes such as replication, transcription or nucleosome assembly.