Ubiquitin-Independent Degradation


Part of our lab studies the uncommon means utilized by the HCMV pp71 protein to induce the degradation of its substrates: pp71 induces the proteasome-dependent, ubiquitin-independent degradation of cellular proteins such as Rb and Daxx.  


Our most recent degradation paper:

Winkler, L.L., Hwang, J., and Kalejta, R.F.  (2013)  Ubiquitin-independent proteasomal degradation of tumor suppressors by human cytomegalovirus pp71 requires the 19S regulatory particle.  J. Virol. 87, 4665-4671.  PMCID: PMC3624388  pdf >publications

A classic degradation paper from our lab:

Hwang, J., and Kalejta, R.F., (2007)  Proteasome-dependent, ubiquitin-independent degradation of Daxx by the viral pp71 protein in human cytomegalovirus-infected cells.  Virology 367, 334-338.  PMCID: none pdf   >publications

One of our degradation review articles:

Hwang, J., Winkler, L., and Kalejta, R.F.  (2011)  Ubiquitin-independent proteasomal degradation during oncogenic viral infections.  Biochim. Biophys. Acta 1816 (2) 147-157.  PMCID:  PMC3193896 pdf   >publications


 

The ubiquitin-proteasome system is the primary intracellular machinery responsible for elimination of unfolded proteins and for the selective destruction of regulatory proteins involved in a wide range of cellular processes. Substrates are normally targeted to the 26S proteasome via polyubiquitination on internal lysine residues.  Polyubiquitin chains mediate the binding of targeted proteins to the proteasome and assist in their unfolding, but are removed from the substrate prior to proteasomal degradation.  The actual substrate for proteasomal degradation is a partially denatured, non-ubiquitinated protein.  Purified 20S and 26S proteasomes can degrade nonubiquitinated, denatured substrates in vitro, indicating that, if a protein can be delivered to the proteasome in a denatured or partially unfolded state, ubiquitination is not required for its degradation. 

The pp71-mediated degradation of the Rb and Daxx proteins occurs through a proteasome-dependent, ubiquitin-independent pathway, the mechanism of which is not known.  There are at least a few known ways in which cellular proteins can be degraded by the proteasome in the absence of ubiquitination.  Rb and the cyclin-dependent kinase inhibitor (CKI) p21 are degraded in a ubiquitin-independent manner when the MDM2 protein mediates their association with the 20S catalytic core of the proteasome.  Other cellular proteins are recognized by the REG-γ activator complex leading to their degradation by the 20S proteasome without polyubiquitination.  Finally, conjugation of the ubiquitin-like FAT10 protein to a model substrate rendered it susceptible to proteasome-dependent, ubiquitin-independent degradation.

We have explored substrate conjugation to a ubiquitin-like protein, direct binding to the proteasome, or the requirement for a specific proteasome adaptor as potential mechanisms mediating pp71-induced proteasome-dependent ubiquitin-independent degradation.  Although we showed pp71 does induce the covalent modification of Daxx with the small ubiquitin-like modifier termed SUMO, we also showed this modification is not required for pp71-dependent Daxx degradation.  Likewise, we have been unable to detect stable complexes between pp71 and either intact proteasomes or individual proteasome subunits.  We did however demonstrate the necessity of the 19S regulatory particle, a common proteasome adaptor, for pp71-mediated degradation of Daxx and Rb. 

Daxx regulates viral IE gene expression at the beginning of both lytic and latent infections, and ubiquitin-independent degradation is a rare event in uninfected cells.  Therefore, drugs that target the pp71-mediated ubiquitin-independent proteasomal degradation of Daxx may have strong antiviral activity with limited toxicity.  A deeper understanding of the mechanism through which pp71 induces the ubiquitin-independent degradation of Daxx should help in designing or screening for such drugs.