Prp8 refers to both the Prp8 protein and Prp8 gene. Prp8's name originates from its involvement in pre-mRNA processing. The Prp8 protein is a large, highly conserved, and unique protein that resides in the catalytic core of the

spliceosome 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 specifi ...

and has been found to have a central role in molecular rearrangements that occur there. Prp8 protein is a major central component of the catalytic core in the spliceosome, and the spliceosome is responsible for splicing of

precursor mRNA Precursor or Precursors may refer to: *Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of unr ...

that contains

introns An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of the cistron .e., gene. ...

and

exons An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequence ...

. Unexpressed introns are removed by the spliceosome complex in order to create a more concise mRNA transcript. Splicing is just one of many different

post-transcriptional modification Transcriptional modification or co-transcriptional modification is a set of biological processes common to most eukaryotic cells by which an RNA primary transcript is chemically altered following transcription from a gene to produce a mature, func ...

s that mRNA must undergo before translation. Prp8 has also been hypothesized to be a cofactor in RNA catalysis.

History

The systematic name for the PRP8 protein is YHR165C. Prp8 protein is coded by a single gene in humans with 42 exons. The size of Prp8 ranges between 230-280 kDa depending on the organism. The sequence coding for the Prp8 protein is highly conserved between eukaryotic organisms, with a 61% identity match between humans and yeast in amino acid sequence. The Prp8 gene is located on chromosome VIII in yeast and chromosome 17 in humans.

Role in splicing

Pre-mRNA splicing involves two trans-esterification reactions and attacks by hydroxyl groups within the spliceosome. In these reactions, spliceosomal intron removal is catalyzed by the spliceosome using the same mechanism as

Group II intron Group II introns are a large class of self-catalytic ribozymes and mobile genetic elements found within the genes of all three domains of life. Ribozyme activity (e.g., self- splicing) can occur under high-salt conditions ''in vitro''. However, ...

s. There are five key small nuclear RNA-protein complexes (

snRNP snRNPs (pronounced "snurps"), or small nuclear ribonucleoproteins, are RNA-protein complexes that combine with unmodified pre-mRNA and various other proteins to form a spliceosome, a large RNA-protein molecular complex upon which splicing of pre- ...

) involved in this process. All of the snRNPs together contribute about 50 proteins to the core spliceosome. The Prp8 gene encodes for a protein that is a central part of the U5 snRNP and the U5-U4/U6 tri-snRNP. The U5-U4/U6 tri-snRNP is involved with Complex B, the pre-catalytic spliceosome, where the U5 snRNP binds to exons at the 5’ end of the mRNA before shifting to introns. The U5 snRNP is involved with Complex C, the catalytic spliceosome, where the U5 snRNP binds to an exon at the 3’ splice site and the

lariat loop A honda knot is the loop knot commonly used in a lasso.John 'Lofty' Wiseman SAS ''Survival Handbook, Revised Edition''; William Morrow Paperbacks (2009) Its round shape, especially when tied in stiff rope, helps it slide freely along the rope it ...

forms. The U5 snRNP is also involved with Complex C*, the post-catalytic spliceosome, where it remains bound to the lariat before the spliced RNA is released and the snRNPs are recycled. Common research methods for studying the structure and functions of Prp8 are co-

immunoprecipitation Immunoprecipitation (IP) is the technique of precipitating a protein antigen out of solution using an antibody that specifically binds to that particular protein. This process can be used to isolate and concentrate a particular protein from a samp ...

and

Western blot The western blot (sometimes called the protein immunoblot), or western blotting, is a widely used analytical technique in molecular biology and immunogenetics to detect specific proteins in a sample of tissue homogenate or extract. Besides detect ...

analysis. The structure of Prp8 includes a

RNA recognition motif RNA recognition motif, RNP-1 is a putative RNA-binding domain of about 90 amino acids that are known to bind single-stranded RNAs. It was found in many eukaryotic proteins. The largest group of single strand RNA-binding protein is the eukaryoti ...

, a MPN / JAB ubiquitin-binding domain near the C-terminus, and a nuclear localization signal (NLS) that tags the protein to be moved to the cell nucleus. The crystal structure of Prp8 protein (residues 885–2413) reveals tightly associated domains that resemble an intron reverse transcriptase and a type II restriction endonuclease. This implies that Prp8 might play roles similar to both the creation of cDNA and in cutting the DNA during splicing. Slide1-structure

Prp8 is also more involved with maintaining proper conformation of the bound RNA cofactors and substrates of the splicing reaction. Prp8, along with two other U5 snRNP proteins, helps to activate the spliceosome and form its catalytic active center. It has been proposed that GTP

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

results in a rearrangement of Prp8 that releases the U1 and U4 snRNPs and is responsible for this activation of the catalytic core of the spliceosome. Prp8 performs a scaffold-like function in the spliceosome and holds onto many of the interacting substrates and subunits. It has been cross-linked at both the 3’ and 5’ splice sites in mRNA. Due to these structural elements, it has been assumed that Prp8 may have evolved from inactivated

retroelement Retrotransposons (also called Class I transposable elements or transposons via RNA intermediates) are a type of genetic component that copy and paste themselves into different genomic locations (transposon) by converting RNA back into DNA through ...

s of

reverse transcriptase A reverse transcriptase (RT) is an enzyme used to generate complementary DNA (cDNA) from an RNA template, a process termed reverse transcription. Reverse transcriptases are used by viruses such as HIV and hepatitis B to replicate their genomes, ...

s, with the snRNPs replacing the catalytic domains of self-splicing ancestors.

Mutation and disease

Deficiencies

Prp8 mutation has been linked to the human disease

Retinitis Pigmentosa Retinitis pigmentosa (RP) is a genetic disorder of the eyes that causes loss of vision. Symptoms include trouble seeing at night and decreasing peripheral vision (side and upper or lower visual field). As peripheral vision worsens, people may ...

causing vision loss, especially progressing into adulthood. This autosomal dominant affliction results with degeneration of the photoreceptors of the retina of the eye. This disorder is caused by mutations in the C-terminus. Retinitis Pigmentosa results from nine

missense mutation In genetics, a missense mutation is a point mutation in which a single nucleotide change results in a codon that codes for a different amino acid. It is a type of nonsynonymous substitution. Substitution of protein from DNA mutations Missense m ...

s in the last exon of the mature mRNA result with changes in seven highly conserved amino acids. Studies in yeast indicate that mutation of the C-terminus affects interactions with Brr2p, a

helicase Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separatin ...

responsible for necessary function for the unwinding of the U1 snRNA/5’SS and U4/U6 RNA helices.

Phenotype mutations of Prp8 across species

''Caenorhabditis elegans'' Prp8 has been linked to reproduction and development.

RNAi RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by o ...

, or RNA Interference, was used to knockout Prp8. This resulted in a high level of sterility, a clear body, and protruding vulva, all phenotypical expressions linked to reproduction and development. Mouse Prp8 mutation has resulted in

(see above). Yeast Prp8 mutation results in a U5 snRNP maturation defect. The U5 snRNAP component of the

splicesosome 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 specifi ...

is necessary to bind to the 5' and 3' exons during pre-mRNA splicing. Mutations with this subunit correlate to reduced or inaccurate editing of RNA. In severe cases, mutations in Prp8 can lead to cell death.

References