UBIQUITIN LIGASES Inhibited and Localized by a Pseudosubstrate

The nuclear factor B (NF-B) pathway is activated upon the phosphorylation-dependent ubiquitination and degradation of the inhibitor of B (IB). The E3 ligase primarly responsible for this ubiquitination is a member of the Skp1, Cul1, F-box protein (SCF)-type E3s, called SCFß^-TrCP. The F-box subunit of this complex is E3RS, which is localized in the nucleus, whereas IB is cytoplasmic. Davis et al. determined that nuclear localization of E3RS was due to interactions with the nuclear protein heterogenous nuclear ribonucleoprotein U (hnRNP U). These interactions occurred through the same WD domain through which E3RS interacted with its substrate IB. Indeed, in vitro these interactions were competitive. The entire functional SCFß^-TrCP was capable of interacting with hnRNP U through binding to the F-box subunit; and despite the ability of SCFß^-TrCP to ubiquitinate IB, hnRNP U was not ubiquitinated under any conditions. Thus, hnRNP U appears to act as a low-affinity pseudosubstrate that can be displaced by the higher-affinity substrate IB. Investigations into the nuclear localization due to this interaction between E3RS and hnRNP U determined that the nuclear localization signal of hnRNP U was essential and that disruption of hnRNP U nuclear localization also disrupted E3RS localization. However, forcing E3RS to the cytoplasm by the attachment of a nuclear export signal did not cause the redistribution of hnRNP U. Mutations that disrupted the hnRNP U and E3RS interaction led to decreased stability of the E3RS complex regardless of whether the proteins were cytoplasmic or nuclear. Thus, the hnRNP U interaction serves three functions: (i) it sequesters the F-box substrate recognition subunit of SCFß^-TrCP in the nucleus, (ii) it inhibits the E3 ligase activity of the complex by acting as a pseudosubstrate, and (iii) it stabilizes the E3RS subunit. How E3RS shuttles in and out of the nucleus to serve as the substrate recognition subunit for SCFß^-TrCP and what processes regulate its movement remain unknown.

M. Davis, A. Hatzubai, J. S. Andersen, E. Ben-Shushan, G. Z. Fisher, A. Yaron, A.Bauskin, F. Mercurio, M. Mann, T. Ben-Neriah, Pseudosubstrate regulation of the SCFß^-TrCP ubiquitin ligase by hnRNP-U. Genes Dev. 16, 439-451 (2002). [Abstract] [Full Text]