Figure 6

SR protein functions other than splicing. (a) mRNA export. SR proteins associate site-specifically with intronless mRNAs, such as histone H2A mRNA [63], to promote their export (left-hand side). The export machinery is as yet unknown. For intron-containing pre-mRNAs (right-hand side), SR protein association with the spliced mRNA has also been suggested to mediate nuclear export through interactions with the RNA export factor ALY/REF and Tip-containing protein (TAP). (b) Translation initiation. Interactions between mRNA-bound SF2/ASF and the protein kinase mTOR trigger phosphorylation of 4E-BP (eIF4E-binding protein). In its phosphorylated form 4E-BP dissociates from the translation initiation factor eIF4E, thereby releasing eIF4E and activating initiation of cap-dependent translation (green arrow) [71].