Diverse fate of ubiquitin chain moieties: The proximal is degraded with the target, and the distal protects the proximal from removal and recycles

다국어 초록 (Multilingual Abstract)

<P><B>Significance</B></P><P>The canonical targeting signal for degrading proteins by the ubiquitin (Ub) system—a chain composed of multiple Ub moieties—has remained a mystery. The structure of the proteasome, the enzyme that recognizes the signal and degrades the target substrate cannot explain why such a long chain is needed. To better understand this problem, we synthesized α-globin to which chains with different number of Ubs were attached. In long adducts, the proximal Ub remains on the substrate, likely securing its attachment to the proteasome, and is degraded with it. The distal Ub protects the proximal from removal by deubiquitinating enzymes and is then removed and recycled. In short adducts, the Ub moieties are rapidly removed, and the substrate remains stable.</P><P>One of the enigmas in the ubiquitin (Ub) field is the requirement for a poly-Ub chain as a proteasomal targeting signal. The canonical chain appears to be longer than the distance between the two Ub-binding proteasomal receptors. Furthermore, genetic manipulation has shown that one receptor subunit is sufficient, which suggests that a single Ub can serve as a degradation signal. To shed light on this mystery, we chemically synthesized tetra-Ub, di-Ub (K<SUP>48</SUP>-based), and mono-Ub adducts of HA-α-globin, where the distal or proximal Ub moieties were tagged differentially with either Myc or Flag. When incubated in a crude cell extract, the distal Ub moiety in the tetra-Ub adduct was mostly removed by deubiquitinating enzymes (DUBs) and reconjugated to other substrates in the extract. In contrast, the proximal moiety was most likely degraded with the substrate. The efficacy of degradation was proportionate to the chain length; while tetra-Ub globin was an efficient substrate, with mono-Ub globin, we observed rapid removal of the Ub moiety with almost no degradation of the free globin. Taken together, these findings suggest that the proximal moieties are necessary for securing the association of the substrate with the proteasome along the proteolytic process, whereas the distal moieties are important in protecting the proximal moieties from premature deubiquitination. Interestingly, when the same experiment was carried out using purified 26S proteasome, mono- and tetra-Ub globin were similarly degraded, highlighting the roles of the entire repertoire of cellular DUBs in regulating the degradation of proteasomal substrates.</P>