Cbl-b interacts with ubiquitinated proteins; differential functions of the UBA domains of c-Cbl and Cbl-b

Cbl proteins are ubiquitin protein ligases, which ubiquitinate activated tyrosine kinases and target them for degradation. Both c-Cbl and Cbl-b have an ubiquitin associated (UBA) domain at their C-terminal end. We observed that high molecular weight ubiquitinated proteins constitutively coimmunoprecipitated with transfected and endogenous Cbl-b, but not c-Cbl. The binding site for these ubiquitinated proteins was mapped to the UBA domain of Cbl-b (UBAb). GST-fusion proteins containing the UBAb interacted with ubiquitinated proteins and polyubiquitin chains in vitro, whereas those containing the UBA domain of c-Cbl (UBA(c)) did not. The UBAb had a much greater affinity for polyubiquitin chains than for monoubiquitin. Analysis of the UBA(b) and UBA(c) demonstrate that the affinity for ubiquitin is determined by multiple amino-acid differences between the two domains. Overexpression of the UBA(b), but not overexpression of the UBA(c), inhibited a variety of ubiquitin-mediated processes such as degradation of ubiquitinated proteins (i.e. EGFR, Mdm-2, and Siah-1). This in vivo result is consistent with the differences in ubiquitin binding observed in vitro between the UBA(b) and UBA(c). This difference in ubiquitin-binding may reflect distinct regulatory functions of c-Cbl and Cbl-b.