CRL3IBTK Regulates the Tumor Suppressor Pdcd4 through Ubiquitylation Coupled to Proteasomal Degradation

The human inhibitor of Bruton's tyrosine kinase isoform alpha (IBtk alpha) is a BTB protein encoded by the IBTK gene, which maps to chromosomal locus 6q14.1, a mutational hot spot in lymphoproliferative disorders. Here, we demonstrate that IBtk alpha forms a CRL3IBTK complex promoting its self-ubiquitylation. We identified the tumor suppressor Pdcd4 as IBtk alpha interactor and ubiquitylation substrate of CRL3(IBTK) for proteasomal degradation. Serum-induced degradation of Pdcd4 required both IBtk alpha and Cul3, indicating that CRL3(IBTK) regulated the Pdcd4 stability in serum signaling. By promoting Pdcd4 degradation, IBtk alpha counteracted the suppressive effect of Pdcd4 on translation of reporter luciferase mRNAs with stem-loop structured or unstructured 5'-UTR. IBtk alpha depletion by RNAi caused Pdcd4 accumulation and decreased the translation of Bcl-xL mRNA, a well known target of Pdcd4 repression. By characterizing CRL3(IBTK) as a novel ubiquitin ligase, this study provides new insights into regulatory mechanisms of cellular pathways, such as the Pdcd4-dependent translation of mRNAs.