Gene synthesis, expression, structures, and functional activities of site-specific mutants of ubiquitin

To study the structure and function of ubiquitin we have chemically synthesized a ubiquitin gene that encodes the amino acid sequence of animal ubiquitin, inserting a series of restriction enzyme sites that divide the gene into eight "mutagenesis modules." A series of site-specific mutations were constructed to selectively perturb various regions of the molecule. The mutant genes were expressed in a large quantity of Escherichia coli, and the modified proteins were purified. To determine the structural effects of the amino acid substitutions, the solution structure of ubiquitin was investigated by two-dimensional NMR and each of the mutant proteins were screened for structural perturbations. With one exception, virtually no changes were seen other than at the point of mutation. Functional studies of the mutant proteins with the ubiquitin-activating enzyme E1 and in the reticulocyte protein degradation assay were used to identify regions of the molecule important to ubiquitin's activity in intracellular proteolysis.