c-Cbl is not required for ERK1/2-dependent degradation of BimEL

Bim(EL) the most abundant Bim splice variant, is subject to ERK1/2-catalysed phosphorylation, which targets it for ubiquitination and proteasome-dependent destruction. In contrast, inactivation of ERK1/2, following withdrawal of survival factors, promotes stabilization of Bim(EL). It has been proposed that the RING finger protein Cbl binds to Bim(EL) and serves as its E3 ubiquitin ligase. However, this is controversial since most Cbl substrates are tyrosine phosphoproteins and yet Bim(EL) is targeted for destruction by ERK1/2-catalysed serine phosphorylation. Consequently, a role for Cbl could suggest a second pathway for Bim(EL) turnover, regulated by direct tyrosine phosphorylation, or could suggest that Bim(EL) is a coincidence detector, requiring phosphorylation by ERK1/2 and a tyrosine kinase. Here we show that degradation of Bim(EL) does not involve its tyrosine phosphorylation; indeed, Bim(EL) is not a tyrosine phosphoprotein. Furthermore, Bim(EL) fails to interact with Cbl and growth factor-stimulated, ERK1/2-dependent Bim(EL) turnover proceeds normally in Cbl-containing or Cbl-/- fibroblasts. These results indicate that Cbl is not required for ERK1/2-dependent Bim(EL) turnover in fibroblasts and epithelial cells and any role it has in other cell types is likely to be indirect.