RNA splicing is a process in molecular biology where a newly-made precursor messenger RNA (pre-mRNA) transcript is transformed into a mature messenger RNA (mRNA). It works by removing all the introns and splicing back together exons. For nuclear-encoded genes, splicing occurs in the nucleus either during or immediately after transcription. For those eukaryotic genes that contain introns, splicing is usually needed to create an mRNA molecule that can be translated into protein. For many eukaryotic introns, splicing occurs in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs). There exist self-splicing introns, that is, ribozymes that can catalyze their own excision from their parent RNA molecule. The process of transcription, splicing and translation is called gene expression, the central dogma of molecular biology.
Alternative splicing, or alternative RNA splicing, or differential splicing, is an alternative splicing process during gene expression that allows a single gene to code for multiple proteins. In this process, particular exons of a gene may be included within or excluded from the final, processed messenger RNA (mRNA) produced from that gene. This means the exons are joined in different combinations, leading to different (alternative) mRNA strands. Consequently, the proteins translated from alternatively spliced mRNAs will contain differences in their amino acid sequence and, often, in their biological functions.
A spliceosome is a large ribonucleoprotein (RNP) complex found primarily within the nucleus of eukaryotic cells. The spliceosome is assembled from small nuclear RNAs (snRNA) and numerous proteins. Small nuclear RNA (snRNA) molecules bind to specific proteins to form a small nuclear ribonucleoprotein complex, which in turn combines with other snRNPs to form a large ribonucleoprotein complex called a spliceosome. The spliceosome removes introns from a transcribed pre-mRNA, a type of primary transcript. This process is generally referred to as splicing. An analogy is a film editor, who selectively cuts out irrelevant or incorrect material from the initial film and sends the cleaned-up version to the director for the final cut.
SR proteins are a conserved family of proteins involved in RNA splicing. SR proteins are named because they contain a protein domain with long repeats of serine and arginine amino acid residues, whose standard abbreviations are "S" and "R" respectively. SR proteins are ~200-600 amino acids in length and composed of two domains, the RNA recognition motif (RRM) region and the RS domain. SR proteins are more commonly found in the nucleus than the cytoplasm, but several SR proteins are known to shuttle between the nucleus and the cytoplasm.
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.
Polypyrimidine tract-binding protein, also known as PTB or hnRNP I, is an RNA-binding protein. PTB functions mainly as a splicing regulator, although it is also involved in alternative 3' end processing, mRNA stability and RNA localization. Two 2020 studies have shown that depleting PTB mRNA in astrocytes can convert these astrocytes to functional neurons. These studies also show that such a treatment can be applied to the substantia nigra of mice models of Parkinson's disease in order to convert astrocytes to dopaminergic neurons and as a consequence restore motor function in these mice.
Heterogeneous nuclear ribonucleoprotein A1 is a protein that in humans is encoded by the HNRNPA1 gene. Mutations in hnRNP A1 are causative of amyotrophic lateral sclerosis and the syndrome multisystem proteinopathy.
Heterogeneous nuclear ribonucleoprotein K is a protein that in humans is encoded by the HNRNPK gene. It is found in the cell nucleus that binds to pre-messenger RNA (mRNA) 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 polymerase II, the protein responsible for most gene transcription. The relative affinities of the proteins for DNA and RNA vary with solution conditions and are inversely correlated, so that conditions promoting strong DNA binding result in weak RNA binding.
Heterogeneous nuclear ribonucleoprotein U is a protein that in humans is encoded by the HNRNPU gene.
Poly(rC)-binding protein 2 is a protein that in humans is encoded by the PCBP2 gene.
The perinucleolar compartment (PNC) is a subnuclear body characterized by its location at the periphery of the nucleolus. The PNC participates in the patterned compartmentalization inside the nucleus to organize the specialized functions. It is almost exclusively found in oncogenic cells and enriched with RNA binding proteins as well as RNA polymerase III transcripts.
Heterogeneous nuclear ribonucleoprotein D0 (HNRNPD) also known as AU-rich element RNA-binding protein 1 (AUF1) is a protein that in humans is encoded by the HNRNPD gene. Alternative splicing of this gene results in four transcript variants.
Heterogeneous nuclear ribonucleoproteins C1/C2 is a protein that in humans is encoded by the HNRNPC gene.
Splicing factor, proline- and glutamine-rich is a protein that in humans is encoded by the SFPQ gene.
Heterogeneous nuclear ribonucleoprotein F is a protein that in humans is encoded by the HNRNPF gene.
Heterogeneous nuclear ribonucleoprotein H is a protein that in humans is encoded by the HNRNPH1 gene.
Heterogeneous nuclear ribonucleoprotein L is a protein that in humans is encoded by the HNRNPL gene.
Heterogeneous nuclear ribonucleoprotein A/B, also known as HNRNPAB, is a protein which in humans is encoded by the HNRNPAB gene. Although this gene is named HNRNPAB in reference to its first cloning as an RNA binding protein with similarity to HNRNP A and HNRNP B, it is not a member of the HNRNP A/B subfamily of HNRNPs, but groups together closely with HNRNPD/AUF1 and HNRNPDL.
Heterogeneous nuclear ribonucleoprotein H3 is a protein that in humans is encoded by the HNRNPH3 gene.
Splicing factor proline and glutamine rich is a protein that in humans is encoded by the SFPQ gene.
