SYNTHETIC, STRUCTURAL AND BIOLOGICAL STUDIES OF THE UBIQUITIN SYSTEM - CHEMICALLY SYNTHESIZED AND NATIVE UBIQUITIN FOLD INTO IDENTICAL 3-DIMENSIONAL STRUCTURES

The solid-phase chemical synthesis of ubiquitin produced a molecule wi th physicochemical properties similar to those of the natural protein. We have grown crystals of this synthetic ubiquitin and performed an X -ray analysis at 1.8 Angstrom resolution in order to compare the synth etic protein with the known natural structure. The crystals were isomo rphous with those of the natural protein, the R-factor between them be ing 7.1%. Difference Fourier analysis shows that the synthetic and nat ural structures are indistinguishable. The co-ordinates of the natural ubiquitin (IUBQ) were used as the starting point for restrained least -squares refinement (TNT program) against the synthetic X-ray data. Th e refinement converged to R = 16.5% and the resulting model did not ch ange when refined against natural ubiquitin X-ray data (R = 18.7 %), F rom both the refinement and featureless difference Fourier synthesis, we conclude that the synthetic and natural protein structures are iden tical. A short discussion about the uses of proteins with 'non-standar d' amino acid residues is included.