Protein composition of human mRNPs spliced in vitro and differential requirements for mRNP protein recruitment

The deposition of proteins onto newly spliced mRNAs has far reaching consequences for their subsequent metabolism. We affinity-purified spliced human mRNPs under physiological conditions from HeLa nuclear extract and present the first comprehensive inventory of their protein composition as determined by mass spectrometry. Several proteins previously not known to be mRNP-associated were detected, including the DEAD-box helicases DDX3, DDX5, and DDX9, and the ELG, hNHN1, BCLAF1, and TRAP150 proteins. The association of some of the newly identified mRNP proteins was shown to be splicing-dependent, but not to require EJC formation. Initial recruitment of EJC proteins to the spliceosome did not require an EJC binding platform at the -20/24 region of the 5' exon. Finally, while recruitment of EJC proteins and stable EJC formation were not dependent on the cap binding complex, several of the newly identified mRNP proteins required the latter for their association with mRNPs. These results provide novel insights into the composition of spliced mRNPs and the requirements for the association of mRNP proteins with the newly spliced mRNA.