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Heterogeneous nuclear ribonucleoproteins (hnRNPs) are complexes of RNA and protein present in the cell nucleus during gene transcription and subsequent post-transcriptional modification of the newly synthesized RNA (pre-mRNA). The presence of the proteins bound to a pre-mRNA molecule serves as a signal that the pre-mRNA is not yet fully processed and therefore not ready for export to the cytoplasm. Since most mature RNA is exported from the nucleus relatively quickly, most RNA-binding protein in the nucleus exist as heterogeneous ribonucleoprotein particles. After splicing has occurred, the proteins remain bound to spliced introns and target them for degradation.
Contents
The proteins involved in the hnRNP complexes are collectively known as heterogeneous ribonucleoproteins. They include protein K and polypyrimidine tract-binding protein (PTB), which is regulated by phosphorylation catalyzed by protein kinase A and is responsible for suppressing RNA splicing at a particular exon by blocking access of the spliceosome to the polypyrimidine tract.
Functions
- Prevent folding of pre-mRNA into secondary structures that may inhibit its interactions with other proteins.
- May associate with the splicing apparatus.
- Transport of mRNA out of the nucleus.
The association of a pre-mRNA molecule with a hnRNP particle prevents formation of short secondary structures dependent on base pairing of complementary regions, thereby making the pre-mRNA accessible for interactions with other proteins.
Examples
Human genes encoding heterogeneous nuclear ribonucleoproteins include: