The Basic Subdomain of the c‐Jun Oncoprotein

The structural properties of the basic subdomain of the basic zipper (bZIP) protein c‐Jun were examined by joint means of 1 H‐NMR, CD and Fourier‐transform infrared (FTIR) spectroscopies. The basic subdomain (residues 252–281 in c‐Jun) is responsible for sequence‐specific recognition of DNA. A modified basic subdomain bSD (residues 1–35) and its N‐terminal part and C‐terminal part fragments (NP, residues 1–19; and, CP, residues 16–35) were prepared by solid‐phase synthesis and purified by HPLC. In aqueous solution, in the absence of DNA, bSD behaved mostly as an unstructured peptide characterized by only 5% a helix. However, upon mixing bSD and a specific DNA fragment, i.e. a CRE(cAMP‐responsive element)‐containing hexadecanucleotide, the α helix was stabilized to an extent of 20% at 20 °C or 35% at 2%C. At the same time, no significant change could be detected in the DNA spectra. Addition of trifluoroethanol to an aqueous bSD sample resulted in an increase of the α‐helix content so that about 60% of α helix was found at a ratio of 75% trifluoroethanol (20°C). These effects were reflected in both CD and FTIR measurements. Changes shown by the CD spectra during the process suggested a mechanism dominated by a two‐state helix/unordered transition. NMR data, namely α H chemical shifts, NOE cross‐peaks and N H temperature coefficients provided indications for extended or nascent helix structures within four short stretches dispersed along the sequence for c‐Jun bSD, contrasting with the unique and continuous stretch reported for Gcn4 (yeast general control protein 4) bSD in aqueous solution. Trifluoroethanol stabilized the α‐helix structure mainly at these four sites. The malleability of the basic subdomain of c‐Jun was emphasized in relation to its ability to fit the DNA helix in adopting an α‐helix structure. The complex formation apparently requires substantial conformational change from the peptide and only little from the oligonucleotide.