Nuclear speckles (NSs) serve as splicing factor storage sites. In this study, we 
unexpectedly found that many endogenous intronless mRNAs, which do not undergo 
splicing, associate with NSs. These associations do not require transcription, 
polyadenylation, or the polyA tail. Rather, exonic splicing enhancers present in 
intronless mRNAs and their binding partners, SR proteins, promote intronless 
mRNA localization to NSs. Significantly, speckle targeting of mRNAs promotes the 
recruitment of the TREX export complex and their TREX-dependent nuclear export. 
Furthermore, TREX, which accumulates in NSs, is required for releasing 
intronless mRNAs from NSs, whereas NXF1, which is mainly detected at nuclear 
pores, is not. Upon NXF1 depletion, the TREX protein UAP56 loses speckle 
concentration but coaccumulates with intronless mRNAs and polyA RNAs in the 
nucleoplasm, and these RNAs are trapped in NSs upon UAP56 codepletion. We 
propose that the export-competent messenger RNP assembly mainly occurs in NSs 
for intronless mRNAs and that entering NSs serves as a quality control step in 
mRNA export.