Gain of Additional BIRC3 Protein Functions through 3ʹ-UTR-Mediated Protein Complex Formation

Alternative 3ʹ untranslated regions (3ʹ UTRs) are widespread, but their functional roles are largely unknown. We investigated the function of the long BIRC3 3ʹ UTR, which is upregulated in leukemia. The 3ʹ UTR does not regulate BIRC3 protein localization or abundance but is required for CXCR4-mediated B cell migration. We established an experimental pipeline to study the mechanism of regulation and used mass spectrometry to identify BIRC3 protein interactors. In addition to 3ʹ-UTR-independent interactors involved in known BIRC3 functions, we detected interactors that bind only to BIRC3 protein encoded from the mRNA with the long 3ʹ UTR. They regulate several functions, including CXCR4 trafficking. We further identified RNA-binding proteins differentially bound to the alternative 3ʹ UTRs and found that cooperative binding of Staufen and HuR mediates 3ʹ-UTR-dependent complex formation. We show that the long 3ʹ UTR is required for the formation of specific protein complexes that enable additional functions of BIRC3 protein beyond its 3ʹ-UTR-independent functions. Lee and Mayr show that the E3 ligase BIRC3 has several different functions that are exclusively accomplished by BIRC3 protein encoded from the mRNA transcript containing the long, but not the short, 3ʹ UTR. 3ʹ-UTR-dependent protein functions are caused by BIRC3 protein complexes that require the long 3ʹ UTR for their formation.