Thus, the removal of monoubiquitinated Hrs from endosomal membrane could facilitate the clearance of the nonfunctional adapter and its replacement with nonubiquitinated and sorting-competent Hrs, as recently proposed in the case of growth factor receptors . However, while in the same system Hrs is subjected to ubiquitin-dependent degradation, upon FcεRI engagement we have not detected any significant reduction in Hrs protein level consistent with the absence of polyubiquitinated Hrs species. Thus,
in our system Hrs ubiquitination would mainly serve to relocate buy Z-VAD-FMK Hrs from endosomes to the cytosol. All together our findings are compatible with the following scenario: upon antigen stimulation
ubiquitinated selleck chemicals FcεRI complexes are recognized by Hrs that becomes a substrate for Syk and Cbl enzymatic activities. We did not address the order in which Hrs phosphorylation and ubiquitination occur; however it is likely that Syk-induced Hrs phosphorylation occurs at the endosomal membrane and precedes Hrs ubiquitination. Monoubiquitinated Hrs is then removed from endosomal sorting sites allowing its replacement with non-ubiquitinated Hrs that may need to be tyrosine phosphorylated to interact with other endocytic adapters in order to ensure an efficient transport of ubiquitinated cargos. In this scenario, Hrs monoubiquitination would serve to relocate Hrs from endosomes to the cytosol, without promoting degradative events. Although additional experiments are required to Selleck Hydroxychloroquine validate our model, we demonstrate for the first time that engagement of an IR, namely FcεRI, has the potential to trigger Hrs phosphorylation and monoubiquitination, and that both inducible modifications require Syk kinase activity. From a broader cell biological perspective, this finding could be extended to include other IRs, such as the TCR and BCR, providing a novel regulatory mechanism used by the Syk family kinases to attenuate immune responses in mammalian
cells. The anti-FcεRI β subunit mAb (JRK) was kindly provided by Dr. J.-P. Kinet (Beth Israel Deaconess Medical Center, Boston, MA, USA). The anti-FcεRI γ subunit polyclonal Ab and the anti-pTyr 4G10 mAb were from UBI (Lake Placid, NY). Rabbit anti-Syk (N-19 and C-20), anti-Hrs (M-79), anti-Cbl (C-15) Abs, and anti-Hrs (D-3 and C-7) mAbs were from Santa Cruz Biotechnology (Santa Cruz, CA). Anti-phospho 334 Hrs Ab was from Assay Biotech (San Francisco, CA). Anti-DNP-specific mouse IgE (clone SPE-7), anti-actin (AC-15), and anti-β tubulin (Tub2.1) mAbs and all chemicals were from Sigma-Aldrich (Milan, Italy). The anti-Ub FK2 and FK1 mAbs were from Enzo Life Sciences (Exeter, United Kingdom). Lyso-Tracker Red was from Molecular Probes (Eugene, OR, USA). Purified and FITC-conjugated rat anti-IgE mAbs were from BD Biosciences (San Jose, CA, USA).