Expression of Sunflower Homeodomain Containing Proteins inEscherichia coli:Purification and Functional Studies

Complementary DNA sequences encoding different portions of two sunflower homeodomain proteins were cloned in-frame in the expression vectors pRSET and pGEX-3X. When introduced into competentEscherichia colicells and induced, the resulting plasmids directed the expression of large amounts (5–10% of total cellular protein) of the encoded polypeptides. As a rule, fusions in pRSET rendered insoluble proteins, while fusions in pGEX were soluble and could be purified in a single step by selective absorption onto glutathione–agarose beads, followed by elution with free glutathione. The purified proteins showed both glutathioneS-transferase and DNA-binding activity, indicating that they retain their native conformation. The expression–purification protocol that was employed allowed the isolation of up to 0.7 mg of protein per gram of transformed cells. One of the fusion proteins, RH11 (which is a fusion of the homeodomain protein HAHR1 in pRSET), though insoluble, was able to bind DNA when spotted onto a nitrocellulose filter. This protein could also be simply purified in large amounts by electroelution from sodium dodecyl sulfate–polyacrylamide gels and used to elicit antibodies which recognized both the transgenic fusion and the native protein from sunflower nuclei. Our results clearly show that vector choice is a critical parameter for obtaining large amounts of a desired protein for particular purposes.