19. Which one of the following mRNAs is most likely to be exported out of the nucleus?
(1) Spliced RNA associated with the poly A binding and cap binding complex.
(2) Mis-spliced RNA with multiple stop codons, for degradation in cytosol.
(3) Spliced RNA with the associated spliceosomal complex.
(4) Uncapped and unspliced RNA.

The mRNA most likely to be exported out of the nucleus is:

(1) Spliced RNA associated with the poly A binding and cap binding complex.

Explanation:

• Mature, export-competent mRNAs in eukaryotic cells are typically spliced, 5′ capped, and 3′ polyadenylated. These modifications and processing steps are crucial for forming a proper messenger ribonucleoprotein particle (mRNP) that is recognized by the nuclear export machinery.

• The cap binding complex (CBC) at the 5′ end and the poly(A) binding proteins (PABPs) at the 3′ end help recruit export factors such as NXF1 (also called TAP) and associated adaptor proteins, which mediate transport through the nuclear pore complex (NPC) into the cytoplasm.

• Mis-spliced RNAs with premature stop codons are usually retained in the nucleus or targeted for degradation via quality control mechanisms such as nonsense-mediated decay (NMD), preventing their export.

• Spliced RNA still associated with spliceosomal complexes is generally not export-ready because the spliceosome must be fully disassembled before export factors can bind.

• Uncapped and unspliced RNAs lack the necessary processing marks and binding proteins, making them poor substrates for export and often targets for nuclear retention and degradation.

Summary

Only properly processed mRNAs—those that are spliced, capped, and polyadenylated with associated binding proteins—are efficiently exported from the nucleus. This ensures that only mature, functional mRNAs reach the cytoplasm for translation.

Answer: (1) Spliced RNA associated with the poly A binding and cap binding complex.