Soluble expression and purification of tumor suppressor WT1 and its zinc finger domain

Abstract

Full length murine WT1 and its zinc finger domain were separately inserted into Escherichia coli expression vectors with various fusion tags on either terminus by Gateway technology (Invitrogen) and expression of soluble protein was assessed. Fusion proteins including the four zinc finger domains of WT1 were used to optimize expression and purification conditions and to characterize WT1:DNA interactions in the absence of WT1:WT1 interactions. Zinc finger protein for in vitro characterization was prepared by IMAC purification of WT1 residues 321–443 with a thioredoxin–hexahistidine N-terminal fusion, followed by 3C protease cleavage to liberate the zinc fingers and cation exchange chromatography to isolate the zinc fingers and reduce the level of the truncated forms. Titration of zinc finger domain with a binding site from the PDGFA promoter gave a Kd of 100±30nM for the −KTS isoform and 130±40nM for the +KTS isoform. The zinc finger domain was also co-crystallized with a double-stranded DNA oligonucleotide, yielding crystals that diffract to 5.5Å. Using protocols established for the zinc finger domain, we expressed soluble full-length WT1 with an N-terminal thioredoxin domain and purified the fusion protein by IMAC. In electro-mobility shift assays, purified full-length WT1 bound double-stranded oligonucleotides containing known WT1 binding sites, but not control oligonucleotides. Two molecules of WT1 bind an oligonucleotide presenting the full PDGFA promoter, demonstrating that active full-length WT1 can be produced in E. coli and used to investigate WT1 dimerization in complex with DNA in vitro.