Mouse anti-Human RBM39 monoclonal antibody, clone 5H9

Exon 16 of protein 4.1R encodes a spectrin/actin-binding peptide critical for erythrocyte membrane stability. Its expression during erythroid differentiation is regulated by alternative pre-mRNA splicing. A UUUUCCCCCC motif situated between the branch point and the 3' splice site is crucial for inclusion. We show that the UUUU region and the last three C residues in this motif are necessary for the binding of splicing factors TIA1 and Pcbp1 and that these proteins appear to act in a collaborative manner to enhance exon 16 inclusion. This element also activates an internal exon when placed in a corresponding intronic position in a heterologous reporter. The impact of these two factors is further enhanced by high levels of RBM39, whose expression rises during erythroid differentiation as exon 16 inclusion increases. TIA1 and Pcbp1 associate in a complex containing RBM39, which interacts with U2AF65 and SF3b155 and promotes U2 snRNP recruitment to the branch point. Our results provide a mechanism for exon 16 3' splice site activation in which a coordinated effort among TIA1, Pcbp1, and RBM39 stabilizes or increases U2 snRNP recruitment, enhances spliceosome A complex formation, and facilitates exon definition through RBM39-mediated splicing regulation.

The molecular glue that connects two different proteins is a unique but not an entirely new chemical entity. One of the notable research achievements with this molecular class is sequential identification of molecular mechanisms of two immunosuppressive natural products FK506 and rapamycin. FK506 binds to FKBP12 to interact with and inhibit the protein phosphatase calcineurin. Likewise, rapamycin binds to FKBP12 to interact with and inhibit the phosphatidylinositol 3-kinase-related kinase mTOR. Importantly, the molecular glue story has recently been extended to synthetic small molecule space as well. Clinically important myeloma drug lenalidomide and other immunomodulatory imide drugs (IMiDs) were found to hijack the cullin 4 (CUL4)-RING E3 ubiquitin ligase system by binding to the one of DDB1- and CUL4-associated proteins (DCAFs), cereblon (CRBN), to redirect the substrate selectivity of CRBN. In consequence of the small molecule mediated neo-substrate recruitment, IMiDs induce the proteasomal degradation of the transcription factors IKZF1, IKZF3 and casein kinase 1 alpha in a selective manner. We also reported that a series of anticancer sulfonamides such as E7070 (indisulam) and E7820 induce the protein-protein interaction between another DCAF protein DCAF15 and the slicing factor CAPER alpha (also known asRBM39), resulting in the selective proteaosomal degradation of CAPER alpha as a neo-substrate for the E3 ubiquitin ligase. All these findings may indicate a significant opportunity to gain a new insight into molecular glues in drug discovery, leading to further development of new modalities of bi-functional selective protein degraders with target protein knockdown function, represented by PROTAC (Proteolysis Targeting Chimera), Degronimide, and SNIPER (Specific and Nongenetic IAP-dependent Protein Eraser).