The folding of a 93-residue protein, the histidine-phospho-carrier protein ofStreptomyces coelicolor,HPr,hasbeen studied using several biophysical techniques, namely fluo-rescence, 8-anilinonaphthalene-1-sulfate binding, circular dichroism, Fourier transform infrared spectroscopy, gel filtration chromatography and differential scanning calori-metry. The chemical-denaturation behaviour of HPr, fol-lowed by fluorescence, CD and gel filtration, at pH and 25 C, is described as a two-state process, which does not involve the accumulation of thermodynamically stable intermediates. Its conformational stability under those con-ditions isDG¼ ± kcalÆmol )1 (1 kcal ¼), which makes the HPr fromS. coelicolorthe most unstable member of the HPr family described so far. .
