The post-translational attachment of the ubiquitin domain generates a targeting signal that localizes modified proteins to the proteasome, the nucleus, the cytoskeleton and the autophagic system. At least five distinct proteins are known to possess the ubiquitin domain and target modified proteins to different cellular locations. It has been shown that the accumulation of polymeric ubiquitin caused by mutation of deubiquitinating enzymes interferes with the targeting of polyubiquitinated proteins to the proteasome and thus, protein degradation. Polymeric ubiquitin can also arise as a result of the translation of the proubiquitin mRNA. This gene codes for tandem repeats of the ubiquitin domain, and the polyprotein that results must be processed to the monomer by proteolytic cleavage at the mature C-terminus of ubiquitin.Finally, ubiquitin-fusion proteins are also produced in eukaryotic cells and these must be processed at the C-terminus of ubiquitin to release ubiquitin and the C-terminal fusion proteins that are required for ribosome biogenesis and function. It is likely that the cell would have to avoid producing polymeric ubiquitin from these genes to prevent interference with the complex metabolism of post-translational ubiquitination. Based on this expectation, and the observation that the pro-ubiquitin protein is never observed we have hypothesized that processing of the pro-ubiquitin gene product is co-translational. Further, we suspect that the translation of this message is periodically stalled to allow recruitment of the processing enzyme and that this stalling is due to the presence of low abundance codons in each copy of ubiquitin coding sequence. Experiments proposed here will examine these hypotheses and further define cotranslational processing events.
