A Proteolytic Pathway That Recognizes Ubiquitin as a Degradation Signal (*)

Previous work has shown that a fusion protein bearing a “nonremovable” N-terminal ubiquitin (Ub) moiety is short-lived in vivo, the fusion's Ub functioning as a degradation signal. The proteolytic system involved, termed the UFD pathway (Ub fusion degradation), was dissected in the yeast Saccharomyces cerevisiae by analyzing mutations that perturb the pathway. Two of the five genes thus identified, UFD1 and UFD5, function at post-ubiquitination steps in the UFD pathway. UFD3 plays a role in controlling the concentration of Ub in a cell: ufd3 mutants have greatly reduced levels of free Ub, and the degradation of Ub fusions in these mutants can be restored by overexpressing Ub. UFD2 and UFD4 appear to influence the formation and topology of a multi-Ub chain linked to the fusion's Ub moiety. UFD1, UFD2, and UFD4 encode previously undescribed proteins of 40, 110, and 170 kDa, respectively. The sequence of the last 280 residues of Ufd4p is similar to that of E6AP, a human protein that binds to both the E6 protein of oncogenic papilloma viruses and the tumor suppressor protein p53, whose Ub-dependent degradation involves E6AP. UFD5 is identical to the previously identified SON1, isolated as an extragenic suppressor of sec63 alleles that impair the transport of proteins into the nucleus. UFD5 is essential for activity of both the UFD and N-end rule pathways (the latter system degrades proteins that bear certain N-terminal residues). We also show that a Lys Arg conversion at either position 29 or position 48 in the fusion's Ub moiety greatly reduces ubiquitination and degradation of Ub fusions to β-galactosidase. By contrast, the ubiquitination and degradation of Ub fusions to dihydrofolate reductase are inhibited by the Ub but not by the Ub moiety. ufd4 mutants are unable to ubiquitinate the fusion's Ub moiety at Lys, whereas ufd2 mutants are impaired in the ubiquitination at Lys. These and related findings suggest that Ub-Ub isopeptide bonds in substrate-linked multi-Ub chains involve not only the previously identified Lys but also Lys of Ub, and that structurally different multi-Ub chains have distinct functions in Ub-dependent protein degradation.