A Novel Splicing Factor RBM25 in Erythroid Differentiation.

Many protein factors that guide pre-mRNA modification pathways are composed of RNA recognition motif (RRM) domains. In an effort to identify splicing regulators in erythroid cells, we cloned a RRM-containing protein RBM25 and characterized its role in alternative splicing. RBM25 consists of a proline-rich region and a RRM domain at the amino-terminal end, an ER rich domain at the central region, and a PWI domain at the carboxyl terminal end. RBM25 partially co-localized with splicing factor SC35 in nuclear speckles. While both the RRM and PWI domains are diffusely distributed in the nucleoplasm, the ER domain is highly concentrated in nuclear speckles. We examined cellular mRNA targets of RBM25 in HeLa cells and demonstrated that it binds Bcl-x mRNA and affects its alternative splicing. Depletion of RBM25 by RNA interference caused accumulation of anti-apoptotic Bcl-x(L), whereas its up-regulation increased the levels of pro-apoptotic Bcl-x(s). The expression level of RBM25 also correlated with the degree of cell death, further suggesting that it functions in regulating apoptotic factor(s) expression. Apoptosis plays an important role in red cell development; earlier erythroid progenitors are more sensitive to apoptosis while mature erythroblasts are resistant to apoptosis. A significant decrease in RBM25 expression occurs during erythroid differentiation and correlates with resistance to apoptosis. Our results suggest that erythroblasts may acquire resistance to apoptosis during maturation through differential expression of crucial splicing regulators of the apoptotic machinery.