ATP-Dependent Recruitment of Export Factor Aly/REF onto Intronless mRNAs by RNA Helicase UAP56

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Abstract
Loading of export factors onto mRNAs is a key step in gene expression. In vertebrates, splicing plays a role in this process. Specific protein complexes, exon junction complex and transcription/export complex, are loaded onto mRNAs in a splicing-dependent manner, and adaptor proteins such as Aly/REF in the complexes in turn recruit mRNA exporter TAP-p15 onto the RNA. By contrast, how export factors are recruited onto intronless mRNAs is largely unknown. We previously showed that Aly/REF is preferentially associated with intronless mRNAs in the nucleus. Here we show that Aly/REF could preferentially bind intronless mRNAs in vitro and that this binding was stimulated by RNA helicase UAP56 in an ATP-dependent manner. Consistently, an ATP binding-deficient UAP56 mutant specifically inhibited mRNA export in Xenopus oocytes. Interestingly, ATP activated the RNA binding activity of UAP56 itself. ATP-bound UAP56 therefore bound to both RNA and Aly/REF, and as a result ATPase activity of UAP56 was cooperatively stimulated. These results are consistent with a model in which ATP-bound UAP56 chaperones Aly/REF onto RNA, ATP is then hydrolyzed, and UAP56 dissociates from RNA for the next round of Aly/REF recruitment. Our finding provides a mechanistic insight into how export factors are recruited onto mRNAs.