Signal Specific Ubiquitination and Degradation of IkBa
The transcription factor Nuclear Factor kB (NF-kB) is retained in the cytoplasm by the action of its inhibitor IkB. Upon phosphorylation by the IKK complex, IkB is rapidly ubiquitinated and targeted for 26S proteasome mediated degradation, thus liberating NF-kB for transport to its nuclear destination. The current project was initiated to reconstitute this pathway in vitro by using the purified ubiquitination and degradation machinery to degrade IkBa, and activate NF-kB. While signal dependant IkBa ubiquitination was achieved early in the project, this substrate was not degraded by a number of different 26S protein preparations. Instead, an integral or associated isopeptidase activity was observed with each 26S preparation. The development of new 26S protein purification methods has enabled the isolation of highly purified 26S proteins that exhibits low degradative activity towards the ubiquitinated IkBa substrate without excess isopeptidase activity. In an effort to increase substrate degradation, the IkBa ubiquitination reaction was carefully scrutinized. The current literature reports that Ubch5 is the relevant E2 that works in conjunction with the IkBa SCFᔲCP E3 complex, however, Cdc34/Ubc3 can also ubiquitinate IkBa, and may also be a relevant E2. While both E2s carry out in vitro signal dependant ubiquitination of IkBa, the ubiquitin conjugates made by Ubc3 are specific for Lysine-48 linked isopeptide bonds, whereas Ubch5 is able to utilize a variety of ubiquitin surface Lysine residues in isopeptide bond formation. Because K-48 linked ubiquitin conjugates are believed to target substrates for 26S mediated degradation, it was not surprising to find that my 26S proteasome preparations exhibited higher levels of IkBa degradation when ubiquitin conjugation reactions were carried out with Ubc3 instead of Ubch5. Using small interfering RNA to knock down the protein levels of each E2 in vivo, we have found that Ubc3 has no effect on IkBa degradation, whereas the Ubc5/7 double knockdown exhibits partial inhibition of IkBa degradation which is comparable to knocking down the levels of the IkBa E3 specificity factor ᔲCP. The completion of this project has established an in vitro ubiquitination and degradation system that will be instrumental for future studies aimed at determining how the 26S proteasome unfolds and degrades its protein substrates.