Regulation of the ubiquitin-mediated proteolytic pathway: role of the substrate alpha-NH2 group and of transfer RNA.

Degradation of intracellular proteins via the ubiquitin pathway involves several steps. In the initial event, ubiquitin becomes covalently linked to the protein substrate in an ATP-requiring reaction. Following ubiquitin conjugation, the protein moiety of the adduct is selectively degraded with the release of free and reusable ubiquitin. Ubiquitin modification of a variety of protein targets in the cell plays a role in basic cellular functions. Modification of core nucleosomal histones is probably involved in regulation of gene expression at the level of chromatin structure. Ubiquitin attachment to cell surface proteins may play roles in processes of cell-cell interaction and adhesion, and conjugation of ubiquitin to other yet to be identified protein(s) could be involved in the progression of cells through the cell cycle. Despite the considerable progress that has been made in the elucidation of the mode of action and cellular roles of the ubiquitin pathway, many major problems remain unsolved. A problem of central importance is the specificity in the ubiquitin ligation system. Why are certain proteins conjugated and committed for degradation, whereas other proteins are not? A free alpha-NH2 group is an important feature of the protein structure recognized by the ubiquitin conjugation system, and tRNA is required for the conjugation of ubiquitin to selective proteolytic substrates and for their subsequent degradation. These findings can shed light on some of the features of a substrate that render it susceptible to ubiquitin-mediated degradation.