About HNRNP / Heterogeneous Nuclear Ribonucleoprotein:
HNRNP, also known as Heterogeneous nuclear ribonucleoproteins, are complexes of RNA and protein found in the nucleus of cells during gene transcription and subsequent post-transcriptional modification of the newly synthesized RNA (pre-mRNA). Proteins that are bound to a pre-mRNA molecule serve as a single that the pre-mRNA hasn’t been processed yet, and this means that it isn’t ready for transportation to the cytoplasm.
Most RNA-binding proteins in the nucleus exist as heterogeneous ribonucleoprotein particles, with mature RNA being exporting from the nucleus as soon as it is ready. After splicing has occurred, the proteins remain bound to spliced introns and target them for degradation.
hnRNPs are also integral to the 40s subunit of the ribosome, making them important for cytoplasm translation of mRNA. hnRNPs are mainly found in the nucleus thanks to their own nuclear localization sequences (NLS). Immunofluorescence microscopy with hnRNP-specific antibodies shows nucleoplasmic localization of these proteins with little staining in the nucleolus or cytoplasm. This is thanks to its major role in binding to freshly transcribed RNAs. hnRNPs localizes predominantly to the border regions of chromatin to gain access to these nascent RNAs.
Groups of proteins involved in hnRNP complexes are called heterogeneous ribonucleoproteins. These are regulated by phosphorylation catalyzed by protein kinase A and are responsible for suppressing RNA splicing at a particular exon by blocking access of the spliceosome to the polypyrimidine tract, and include protein K and polypyrimidine tract-binding protein (PTB). Interactions between attached hnRNPs may encourage certain splicing combinations while inhibiting others.