We demonstrate

that Hrs, concomitant with its association with Syk, undergoes tyrosine phosphorylation and monoubiquitination in RBL-2H3 mast cells and in mouse BMMCs upon FcεRI engagement, and we identify Syk as the main kinase responsible for Hrs tyrosine phosphorylation in RBL-2H3 cells. Hrs undergoes also monoubiquitination upon antigen stimulation. This result is in line with the finding of Polo et al. [26], which reported the EGF-dependence of Hrs ubiquitination. However, in contrast to previous reports [25, 27], we did not found clear evidence for Talazoparib solubility dmso poly-ubiquitinated forms of Hrs in RBL-2H3 cells. By siRNA knock down of c-Cbl, we demonstrate that in RBL-2H3 cells c-Cbl is required for inducible Hrs monoubiquitination. This finding is consistent with a prior report showing that ectopic expression of WT c-Cbl enhances Hrs ubiquitination upon stimulation with growth factors [28]. Notably, we demonstrate that inducible Hrs monoubiquitination this website requires Syk kinase activity. How Syk and Cbl might act in concert to regulate Hrs monoubiquitination? Syk and Cbl have been previously reported to be constitutively associated in RBL-2H3 cells [30]. FcεRI engagement promotes Syk/Cbl membrane recruitment and the subsequent activation of

both enzymes, being the kinase activity of Syk required for Cbl ligase activity [17]. In this scenario, the combined enzymatic activities of Syk and Cbl can act on Hrs upon its recognition of ubiquitinated receptors. Moreover, Syk-induced Hrs phophorylation might precede and represent a signal for Hrs Axenfeld syndrome monoubiquitination. Finally, we provided evidence that phosphorylation and monoubiquitination of Hrs serve to control its membrane/cytosol localization. We show that upon FcεRI engagement Hrs is present into membrane and cytosolic fractions. However, an increase of Hrs phosphorylation was reproducibly observed only in membranes, suggesting that Syk preferentially phosphorylates Hrs located

into endosomal sorting site. Consistent with this assumption, we observed a predominant localization of Syk in membrane fractions upon receptor engagement. In agreement with these data, previous studies have shown that endosomal localization of Hrs is required for its phosphorylation [23]. Although Hrs does not need to be tyrosine phosphorylated to bind to ubiquitinated cargo proteins [25], the phosphorylation status of Hrs may generate new docking sites that can lead to interaction with other endocytic adapters. We are actually investigating this latter possibility. Interestingly, we also found that monoubiquitinated Hrs forms are preferentially confined on cytosolic fractions. The relocation of ubiquitinated Hrs from membrane to cytosolic compartments may be functionally significant.