ABSTRACT
To clear up the respective roles of the ubiquitin and tail portions, the corresponding genetic segments were separated and expressed using heterologous promoters. Ubiquitin is a 76-amino acid protein found in all eukaryotic cells, either free or covalently attached to other proteins. It shows a remarkable degree of sequence conservation and plays a central role in selective protein degradation. The amino-terminal location of an amino acid is essential for its effect on β-gal half-life. An in vitro proteolytic system has been derived by fractionation from rabbit reticulocytes. The role of N-α-acetylation is not yet clear, but it has been found that N-α-acetylated proteins are degraded in whole reticulocyte lysate by the ubiquitin system. The results could be explained by the existence, in both prokaryotes and eukaryotes, of aminoacyl-tRNA-protein transferases, catalyzing the transfer of specific amino acid residues to the mature N terminus of acceptor proteins.
