Assessment of the transcriptional activation potential of the HMG chromosomal proteins

Chromosomal proteins HMG-14, HMG-17, and HMG-1 are among the most abundant, ubiquitous, and evolutionarily conserved nonhistone proteins. Analysis of their structure reveals features which are similar to those of certain transcription factors. The distribution of charged amino acid residues along the polypeptide chains is asymmetric: positive charges are clustered toward the N-terminal region, while negative charges are clustered toward the C-terminal region. The residues in the C-terminal region have the potential to form alpha helices with negatively charged surfaces. The abilities of HMG-14, -17, and -1 to function as transcriptional activators were studied in Saccharomyces cerevisiae cells expressing LexA-HMG fusion proteins (human HMG-14 and -17 and rat HMG-1) which bind to reporter molecules containing the beta-galactosidase gene downstream from a lexA operator. Fusion constructs expressing deletion mutants of HMG-14, -17, and -1 were also tested. Analysis of binding to the lexA operator with in vitro-synthesized fusion proteins shows that there are more sites for HMG-14, -17, and -1 binding than for LexA binding and that only the fusion constructs which contain the C-terminal, acidic domains of HMG-17 bind the lexA operator specifically. None of the LexA-HMG fusion protein constructs elevate the level of beta-galactosidase activity in transfected yeast cells. Thus, although HMG-14, -17, and -1 are structurally similar to acidic transcriptional activators, these chromosomal proteins do not function as activators in this test system.