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 basophi ls plays an essential role in immunological defense. Upon multivalent antig en binding, FceRI becomes phoshorylated by the proteintyrosine kinase Lyn, as a result of receptor clustering in lipid rafts. Fc epsilon RI has been s hown to be ubiquitinated. Ubiquitination can lead to degradation by proteas omes, but it can also act as a sorting signal to internalize proteins desti ned to the endosomal/lysosomal pathway. We have analyzed whether Fc epsilon RI ubiquitination takes place within rafts. We report biochemical and imag ing evidence in rat basoleukemia cells for the presence of ubiquitinated Fc epsilon RI in clustered rafts upon receptor activation. Moreover, we demon strated that the ubiquitin ligases Chl and Nedd4 colocalize with FceRI patc hes and showed that both ligases become associated with lipid rafts after a ctivation of IgE signaling. Because Cbl is known to interact with the FceRI signaling complex, ubiquitination is likely to be an important parameter r egulating IgE-triggered signaling occurring in rafts.