Nedd4 regulates ubiquitination and stability of the guanine-nucleotide exchange factor CNrasGEF

Cyclic nucleotide ras GEF (CNrasGEF) is a guanine-nucleotide exchange factor previously isolated in a screen for Nedd4-WW domain interacting proteins (Pham, N., Cheglakov, I., Koch, C. A., de Hoog, C. L., Moran, M. F., and Rotin, D. (2000) Curr. Biol. 10, 555-558). It activates Ras in a cAMP-dependent manner and Rap-1 independent of cAMP. Here we show that CNrasGEF is a likely substrate of the ubiquitin protein ligase Nedd4. CNrasGEF possesses two PY motifs at its C terminus that are responsible for binding to Nedd4 in vitro. Moreover, Nedd4 and CNrasGEF co-immunoprecipitate from 293T cells expressing ectopic CNrasGEF and endogenous Nedd4, and this co-immunoprecipitation is abrogated in PY motif-mutated CNrasGEF (CNrasGEFDelta2PY). CNrasGEF is ubiquitinated in cells, and this ubiquitination is augmented upon overexpression of wt-Nedd4 but is inhibited in cells overexpressing a catalytically inactive Nedd4 (Nedd4(CS)) or in cells expressing CNrasGEFDelta2PY, which cannot bind Nedd4. Moreover, pulse-chase experiments have demonstrated that the half-life of CNrasGEF is reduced 5-fold (from approximately 10 to approximately 2 h) in cells co-expressing Nedd4 with CNrasGEF but not with CNrasGEFDelta2PY (t(0.5) approximately 14 h). CNrasGEF is also stabilized in cells co-expressing Nedd4(CS) or following treatment with lactacystin, indicating proteasomal degradation of this protein. Deletion/mutation of the CDC25 domain to abrogate Ras (or Rap-1) binding leads to impaired ubiquitination of CNrasGEF, suggesting that such binding is critical for ubiquitination. Treatment of cells with the cAMP analogue 8-bromo-cAMP does not affect the ability of CNrasGEF to bind Nedd4 nor its level of ubiquitination, suggesting that Ras binding per se and not its activation is the critical step in triggering ubiquitination of CNrasGEF. These results suggest that CNrasGEF is a substrate for Nedd4, which regulates its ubiquitination and stability in cells.