Raft-partitioning of the ubiquitin ligases Cbl and Nedd4 upon IgE-triggered cell signaling

The high affinity receptor for IgE, Fc epsilon RI on mast cells and basophils plays an essential role in immunological defense. Upon multivalent antigen binding, FceRI becomes phoshorylated by the proteintyrosine kinase Lyn, as a result of receptor clustering in lipid rafts. Fc epsilon RI has been shown to be ubiquitinated. Ubiquitination can lead to degradation by proteasomes, but it can also act as a sorting signal to internalize proteins destined to the endosomal/lysosomal pathway. We have analyzed whether Fc epsilon RI ubiquitination takes place within rafts. We report biochemical and imaging evidence in rat basoleukemia cells for the presence of ubiquitinated Fc epsilon RI in clustered rafts upon receptor activation. Moreover, we demonstrated that the ubiquitin ligases Chl and Nedd4 colocalize with FceRI patches and showed that both ligases become associated with lipid rafts after activation of IgE signaling. Because Cbl is known to interact with the FceRI signaling complex, ubiquitination is likely to be an important parameter regulating IgE-triggered signaling occurring in rafts.