Distinct functions of the cap-binding complex in stimulation of nuclear mRNA export.

Cap-binding complex (CBC) associates co-transcriptionally with the cap-structure at the 5'-end of nascent mRNA to protect it from exonucleolytic degradation. Here, we show that CBC promotes the targeting of mRNA export adaptor, Yra1 (that forms transcription-export or TREX complex with THO and Sub2), to the active genes to enhance mRNA export in Saccharomyces cerevisiae Likewise, recruitment of Npl3 (a heteronuclear ribonuclear protein involved in mRNA export via formation of export-competent ribonuclear protein complex or RNP) to the active genes is facilitated by CBC. Thus, CBC enhances targeting of the export factors to promote mRNA export. Such function of CBC is not mediated via THO and Sub2 of TREX, cleavage and polyadenylation factors or Sus1 (that regulates mRNA export via TREX-2 or Transcription-export 2). However, CBC promotes splicing of SUS1 mRNA, and consequently, Sus1 protein level and mRNA export via TREX-2. Collectively, our results support that CBC promotes recruitment of Yra1 and Npl3 to the active genes, independently of THO, Sub2 or cleavage and polyadenylation factors, to enhance mRNA export via TREX and RNP, respectively, in addition to its role in facilitating SUS1 mRNA splicing to increase mRNA export through TREX-2, thus revealing distinct stimulatory functions of CBC in mRNA export.