Heterogeneous ribonucleoprotein particle
Encyclopedia
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 ready for export to the cytoplasm
. After splicing has occurred, the proteins remain bound to spliced intron
s 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
.
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.
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
and protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
present in the cell nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
during gene transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
and subsequent post-transcriptional modification
Post-transcriptional modification
Post-transcriptional modification is a process in cell biology by which, in eukaryotic cells, primary transcript RNA is converted into mature RNA. A notable example is the conversion of precursor messenger RNA into mature messenger RNA , which includes splicing and occurs prior to protein synthesis...
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
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
. After splicing has occurred, the proteins remain bound to spliced intron
Intron
An intron is any nucleotide sequence within a gene that is removed by RNA splicing to generate the final mature RNA product of a gene. The term intron refers to both the DNA sequence within a gene, and the corresponding sequence in RNA transcripts. Sequences that are joined together in the final...
s and target them for degradation.
The proteins involved in the hnRNP complexes are collectively known as heterogeneous ribonucleoproteins. They include protein K
Protein K (gene expression)
Protein K is a human protein found in the cell nucleus that binds to pre-messenger RNA as a component of heterogeneous ribonucleoprotein particles. The simian homolog is known as protein H16. Both proteins bind to single-stranded DNA as well as to RNA and can stimulate the activity of RNA...
and polypyrimidine-tract binding protein (PTB), which is regulated by phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
catalyzed by protein kinase A and is responsible for suppressing RNA splicing
RNA splicing
In molecular biology and genetics, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined. This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation...
at a particular exon
Exon
An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule either after portions of a precursor RNA have been removed by cis-splicing or when two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA...
by blocking access of the spliceosome
Spliceosome
A spliceosome is a complex of snRNA and protein subunits that removes introns from a transcribed pre-mRNA segment. This process is generally referred to as splicing.-Composition:...
to the polypyrimidine tract
Polypyrimidine tract
The polypyrimidine tract is a region of messenger RNA that promotes the assembly of the spliceosome, the protein complex specialized for carrying out RNA splicing during the process of post-transcriptional modification...
.
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:- HNRNPA0HNRNPA0Heterogeneous nuclear ribonucleoprotein A0 is a protein that in humans is encoded by the HNRNPA0 gene.-Further reading:...
, HNRNPA1, HNRNPA1L1, HNRNPA1L2, HNRNPA3HNRPA3Heterogeneous nuclear ribonucleoprotein A3 is a protein that in humans is encoded by the HNRNPA3 gene.-Further reading:...
, HNRNPA2B1HNRPA2B1Heterogeneous nuclear ribonucleoproteins A2/B1 is a protein that in humans is encoded by the HNRNPA2B1 gene.HnRNPA2B1 is an autoantigen in autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus and mixed connective tissue disease... - HNRNPAB
- HNRNPB1
- HNRNPCHNRNPCHeterogeneous nuclear ribonucleoproteins C1/C2 is a protein that in humans is encoded by the HNRNPC gene.It is abnormally expressed in fetuses of both IVF and ICSI, which may contribute to the increase risk of birth defects in these ART.-Function:...
, HNRNPCL1 - HNRNPD, HNRPDLHNRPDLHeterogeneous nuclear ribonucleoprotein D-like, also known as HNRPDL, is a protein which in humans is encoded by the HNRPDL gene.- Function :...
- HNRNPF
- HNRNPH1HNRPH1Heterogeneous nuclear ribonucleoprotein H is a protein that in humans is encoded by the HNRNPH1 gene.-Further reading:...
, HNRNPH2HNRPH2Heterogeneous nuclear ribonucleoprotein H2 is a protein that in humans is encoded by the HNRNPH2 gene.-Further reading:...
, HNRNPH3HNRPH3Heterogeneous nuclear ribonucleoprotein H3 is a protein that in humans is encoded by the HNRNPH3 gene.-Further reading:... - HNRNPK
- HNRNPLHNRNPLHeterogeneous nuclear ribonucleoprotein L is a protein that in humans is encoded by the HNRNPL gene.-Interactions:HNRNPL has been shown to interact with PCBP2, HNRPK and PTBP1.-Further reading:...
, HNRPLL - HNRNPM
- HNRNPRHNRNPRHeterogeneous nuclear ribonucleoprotein R is a protein that in humans is encoded by the HNRNPR gene.-Interactions:HNRNPR has been shown to interact with SMN1 and PRMT1.-Further reading:...
- HNRNPU, HNRNPUL1, HNRNPUL2, HNRNPUL3