S6 permutein shows that the unusual target topology is not responsible for the absence of rigid tertiary structure in de novo protein albebetin

Ribosomal protein S6 from Thermus thermophilus was modified to form the unusual unique topology designed earlier for a de novo protein albebetin, The S6 gene was cloned, sequenced and circularly permutated by means of genetic engineering methods, The permutated gene was expressed in Escherichia coli and the permutein was isolated and investigated by means of circular dichroism, fluorescence spectroscopy and scanning microcalorimetry. The permutated protein revealed a pronounced secondary structure close to that of the wild type S6 protein and a rigid tertiary structure possessing cooperative temperature melting, It means that the unusual new topology of albebetin is compatible with a rigid tertiary structure, it may be realized in natural proteins and it is not responsible for the absence of rigid structure in albebetin. (C) 1997 Federation of European Biochemical Societies.