The RNA-binding protein RNA-binding motif protein 4 (RBM4) modulates alternative splicing of muscle-specific mRNA isoforms during muscle cell differentiation. To better understand the physiological function of RBM4, we exploited a gene knockout strategy in the present study. Mice with targeted disruption of one of the two Rbm4 genes exhibited hyperglycemia coincident with reduced levels of serum insulin and reduced size of pancreatic islets. The embryonic pancreases of Rbm4-deficient mice showed reduced expression or aberrant splicing of many transcripts encoding factors required for pancreas cell differentiation and function. Using pancreatic acinar AR42J cells, we demonstrated that RBM4 promoted insulin gene expression by altering the isoform balance of the transcription factors Isl1 and Pax4 via alternative splicing control. RBM4 overexpression was sufficient to convert AR42J cells into insulin-producing cells. Moreover, RBM4 may mediate glucose-induced insulin expression and insulin receptor isoform switches. These results suggest that RBM4 may have role in promoting pancreas cell differentiation and endocrine function, essentially via alternative splicing regulation. The proteomic complexity of mammalian genomes is greatly expanded by the selective use of exons via alternative splicing. The spatiotemporal expression of alternatively spliced mRNA isoforms contributes substantially to cell differentiation and fate specification and hence influences organogenesis. Reprogramming of alternative splicing during cell differentiation can be achieved by induction of a tissue-specific splicing regulator or by switched expression of one factor to another one that has different or even antagonistic splicing activities. Alternative splicing also provides a mechanism for responding to metabolic prompts (1, 2). Gaining comprehensive insight into alternative splicing remains an imperative goal in the postgenome era.The RNA-binding motif 4 (RBM4) protein has multiple functions in mRNA metabolism; it primarily modulates alterative splicing of precursor mRNAs and regulates mRNA translation (3). We previously reported that RBM4 represses expression of the splicing factor polypyrimidine tract-binding protein (PTB) via alternative splicing-coupled nonsense-mediated mRNA decay during muscle cell differentiation (4). Moreover, RBM4 also modulates the use of alternative exons of many mRNAs encoding muscle differentiation factors or cytoskeletal proteins. Therefore, we deduced that RBM4 promotes myogenesis through its role in altering the repertoire of mRNA isoforms. To further our understanding of the physiological function of RBM4, we attempted to exploit the gene knockout strategy in mice. Previous splicing factor knockout and transgenic studies have revealed the functional consequences of alternative splicing in mammalian development and physiological processes (5). For example, cardiac tissue-specific disruption of the splicing factor gene Srsf1 resulted in abnormal cardiac phenotypes in mice and, moreover, revealed a group of ...

The RNA-binding protein Y14 heterodimerizes with Mago as the core of the exon junction complex during precursor mRNA splicing and plays a role in mRNA surveillance in the cytoplasm. Using the Y14/Magoh heterodimer as bait in a screening for its interacting partners, we identified the protein-arginine methyltransferase PRMT5 as a candidate. We show that Y14 and Magoh, but not other factors of the exon junction complex, interact with the cytoplasmic PRMT5-containing methylosome. We further provide evidence that Y14 promoted the activity of PRMT5 in methylation of Sm proteins of the small nuclear ribonucleoprotein core, whereas knockdown of Y14 reduced their methylation level. Moreover, Y14 overexpression induced the formation of a large, active, and small nuclear ribonucleoprotein (snRNP)-associated methylosome complex. However, Y14 may only transiently associate with the snRNP assembly complex in the cytoplasm. Together, our results suggest that Y14 facilitates Sm protein methylation probably by its activity in promoting the formation or stability of the methylosome-containing complex. We hypothesize that Y14 provides a regulatory link between pre-mRNA splicing and snRNP biogenesis.The RNA-binding protein Y14 is evolutionally conserved in metazoans and participates in mRNA biogenesis (1-4). Y14 contains an RNA recognition motif in the central region, which is involved in the interaction with its stable partner Magoh (5). The C-terminal region of Y14 harboring two consecutive arginine-serine (RS) dipeptides and several arginine and glycine residues is predicted to be less structured but can be post-translationally modified (6). In Drosophila, the Y14/Mago homolog participates in transport and translation control of posterior mRNAs during oogenesis (7,8). In vertebrates, Y14/Mago, together with another heterodimeric factor eIF4AIII/MLN51, constitutes the core of the exon junction complex (EJC), 2 which is a multiprotein complex assembled on spliced mRNAs in a splicing-dependent manner (9, 10). In the EJC, Y14 directly interacts with several other factors, including RNA export factor (REF/Aly), RNPS1, and Upf3 (11). The EJC serves as a platform for binding of the mRNA export receptor Tip-associated protein (TAP) and factors involved in nonsense-mediated mRNA decay (4, 10, 12). Y14 indeed plays an important role in nonsense-mediated mRNA decay (4).We have reported previously that the RG-rich sequences in the C-terminal domain of human Y14 can be methylated, and the RS dipeptides can be phosphorylated (6). In this study, our initial attempt to search for the enzymes or regulators responsible for post-translational modification of Y14 led to the identification of the protein-arginine methyltransferase PRMT5 as a potential interacting factor of Y14. PRMT5 is a type II protein methyltransferase that catalyzes both monomethylation and symmetrical dimethylation (13-15). PRMT5 localizes to both the nucleus and the cytoplasm (14, 16). In the nucleus, PRMT5 methylates transcriptional regulatory factors and may thereby affect ...