Activation of Syk Tyrosine Kinase Is Required for c-Cbl-mediated Ubiquitination of FcεRI and Syk in RBL Cells

• Published in: The Journal of biological chemistry Vol. 277; no. 40; p. 36940
• Main Authors: Rossella Paolini, Rosa Molfetta, Laurie O. Beitz, Juan Zhang, Andrew M. Scharenberg, Mario Piccoli, Luigi Frati, Reuben Siraganian, Angela Santoni
• Format: Journal Article
• Language: English
• Published: American Society for Biochemistry and Molecular Biology 04-10-2002
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M204948200

Engagement of the high affinity receptor for IgE (FcεRI) on mast cells and basophils results in FcεRI β and γ subunits ubiquitination
by an as yet undefined mechanism. Here we show that, upon FcεRI engagement on RBL-2H3 cells Syk undergoes ubiquitination and
Syk kinase activity is required for its own ubiquitination and that of FcεRI β and γ chains. This requirement was demonstrated
by overexpression of Syk wild-type or its kinase-dead mutant in RBL cells or using an Syk-deficient RBL-derived cell line
transfected with wild-type or a kinase inactive form of Syk. We also identify c-Cbl as the E3 ligase responsible for both
Syk and receptor ubiquitination. Furthermore, we demonstrate that Syk controls tyrosine phosphorylation of Syk-associated
Cbl induced after receptor engagement. These data suggest a mutual regulation between Syk and Cbl activities. Finally, we
show that a selective inhibitor of proteasome degradation induces persistence of tyrosine-phosphorylated receptor complexes,
of activated Syk, and of FcεRI-triggered degranulation. Our results provide a molecular mechanism for down-regulation of engaged
receptor complexes by targeting ubiquitinated FcεRI and activated Syk to the proteasome for degradation.