Expression of a highly unstable and insoluble transcription factor in Escherichia coli: purification and characterization of the fork head homolog HNF3 alpha.

The HNF3 alpha transcription factor is highly enriched in adult hepatocytes and activates many liver-specific genes; its expression in hepatic primordia and neurogenic tissue implies a developmental role. HNF3 alpha is a member of a family of proteins, including fork head in Drosophila, which bind DNA at specific sites using a newly defined "winged helix" structural motif. We describe here the expression and purification of full-length HNF3 alpha containing an amino-terminal histidine tag and show that the protein binds DNA with dissociation constant in the sub-nanomolar range. The techniques used to isolate HNF3 alpha should be applicable to other members of the HNF3/fork head family which appear to be generally unstable and insoluble in Escherichia coli.