Heterogeneous ribonucleoprotein particle

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Heterogeneous ribonucleoprotein particles (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 ready for export to the cytoplasm. After splicing has occurred, the proteins remain bound to spliced introns and target them for degradation.

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.

FUNCTION: 1. Prevent folding of pre-mRNA into secondary structures that may inhibit its interactions with other proteins. 2. May associate with the splicing apparatus. 3. Transport of mRNA out of the nucleus.

[edit] References

• Xie J, Lee JA, Kress TL, Mowry KL, Black DL. (2003). Protein kinase A phosphorylation modulates transport of the polypyrimidine tract-binding protein. Proc Natl Acad Sci USA 100(15):8776-81.
• Takimoto M, Tomonaga T, Matunis M, Avigan M, Krutzsch H, Dreyfuss G, Levens D. (1993). Specific binding of heterogeneous ribonucleoprotein particle protein K to the human c-myc promoter, in vitro. J Biol Chem 268(24):18249-58.
• Watson JD, Baker TA, Bell SP, Gann A, Levine M, Losick R. (2004). Molecular Biology of the Gene, ch. 9 and 10. Peason Benjamin Cummings; CSHL Press. 5th ed.