Serine/arginine-rich splicing factor 1 (SRSF1) also known as alternative splicing factor 1 (ASF1), pre-mRNA-splicing factor SF2 (SF2) or ASF1/SF2 is a protein that in humans is encoded by the ''SRSF1'' gene. ASF/SF2 is an essential sequence specific

splicing factor A splicing factor is a protein involved in the removal of introns from strings of messenger RNA, so that the exons can bind together; the process takes place in particles known as spliceosomes. Genes are progressively switched off as we age, and sp ...

involved in pre-mRNA splicing. SRSF1 is the gene that codes for ASF/SF2 and is found on chromosome 17. The resulting splicing factor is a protein of approximately 33 kDa. ASF/SF2 is necessary for all splicing reactions to occur, and influences splice site selection in a concentration-dependent manner, resulting in

alternative splicing 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 ...

. In addition to being involved in the splicing process, ASF/SF2 also mediates post-splicing activities, such as mRNA nuclear export and translation.

Structure

ASF/SF2 is an SR protein, and as such, contains two functional modules: an

arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) and both the am ...

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...

rich region (RS domain), where the bulk of ASF/SF2 regulation takes place, and two RNA recognition motifs (RRMs), through which ASF/SF2 interacts with RNA and other splicing factors. These modules have different functions within general splicing factor function.

Splicing

ASF/SF2 is an integral part of numerous components of the splicing process. ASF/SF2 is required for 5’ splice site cleavage and selection, and is capable of discriminating between cryptic and authentic splice sites. Subsequent lariat formation during the first chemical step of pre-mRNA splicing also requires ASF/SF2. ASF/SF2 promotes recruitment of the U1 snRNP to the 5’ splice site, and bridges the 5’ and 3’ splice sites to facilitate splicing reactions. ASF/SF2 also associates with the U2 snRNP. During the reaction, ASF/SF2 promotes the use of

intron 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. ...

proximal sites and hinders the use of intron distal sites, affecting

. Alternative splicing is affected by ASF/SF2 in a concentration-dependent manner; differing concentrations of ASF/SF2 is a mechanism for alternative splicing regulation, and will result in differing amounts of product isoforms. ASF/SF2 accomplishes this regulation through direct or indirect binding to

exonic splicing enhancer In molecular biology, an exonic splicing enhancer (ESE) is a DNA sequence motif consisting of 6 bases within an exon that directs, or enhances, accurate splicing of heterogeneous nuclear RNA (hnRNA) or pre-mRNA into messenger RNA (mRNA). Introdu ...

(ESE) sequences.

Post-splicing

ASF/SF2, in the presence of elF4E, promotes the initiation of translation of

ribosome Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...

-bound mRNA by suppressing the activity of 4E-BP and recruiting molecules for further regulation of translation. ASF/SF2 interacts with the nuclear export protein TAP in a regulated manner, controlling the export of mature mRNA from the

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...

. An increase in cellular ASF/SF2 also will increase the efficiency of

nonsense-mediated mRNA decay Nonsense-mediated mRNA decay (NMD) is a surveillance pathway that exists in all eukaryotes. Its main function is to reduce errors in gene expression by eliminating mRNA transcripts that contain premature stop codons. Translation of these aberrant ...

(NMD), favoring NMD that occurs before mRNA release from the nucleus over NMD that occurs after mRNA export from the nucleus to the cytoplasm. This shift in NMD caused by increased ASF/SF2 is accompanied by overall enhancement of the pioneer round of translation, through elF4E-bound mRNA translation and subsequent translationally active ribosomes, increased association of pioneer translation initiation complexes with ASF/SF2, and increased levels of active TAP.

Regulation through phosphorylation

ASF/SF2 has the ability to be phosphorylated at the serines in its RS domain by the SR specific protein kinase, SRPK1. SRPK1 and ASF/SF2 form an unusually stable complex of apparent K_d of 50nM. SRPK1 selectively phosphorylates up to twelve serines in the RS domain of ASF/SF2 through a directional and processive mechanism, moving from the C terminus to the N terminus. This multi-phosphorylation directs ASF/SF2 to the nucleus, influencing a number of protein-protein interactions associated with splicing. ASF/SF2's function in export of mature mRNA from the nucleus is dependent on its phosphorylation state; dephosphorylation of ASF/SF2 facilitates binding to TAP, while phosphorylation directs ASF/SF2 to nuclear speckles. Both phosphorylation and dephosphorylation of ASF/SF2 are important and necessary for proper splicing to occur, as sequential phosphorylation and dephosphorylation marks the transitions between stages in the splicing process. In addition, hypophosphorylation and hyperphosphorylation of ASF/SF2 by Clk/Sty can lead to inhibition of splicing.

Biological importance

Stability and fidelity

ASF/SF2 is involved in genomic stability; it is thought that

RNA Polymerase In molecular biology, RNA polymerase (abbreviated RNAP or RNApol), or more specifically DNA-directed/dependent RNA polymerase (DdRP), is an enzyme that synthesizes RNA from a DNA template. Using the enzyme helicase, RNAP locally opens the ...

recruits ASF/SF2 to nascent RNA transcripts to impede formation of mutagenic DNA:RNA hybrid R-loop structures between the transcript and the template DNA. In this way, ASF/SF2 is protecting cells from the potential deleterious effects of transcription itself. ASF/SF2 is also implicated in cellular mechanisms to hinder exon skipping and to ensure splicing is occurring accurately and correctly.

Development and growth

ASF/SF2 has been shown to have a critical function in heart development, embryogenesis, tissue formation, cell motility, and cell viability in general.

Clinical significance

SFRS1 is a proto-oncogene, and thus ASF/SF2 can act as an oncoprotein; it can alter the splicing patterns of crucial cell cycle regulatory genes and suppressor genes. ASF/SF2 controls the splicing of various tumor suppressor genes,

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

s, and kinase receptors, all of which have the potential to be alternatively spliced into oncogenic isoforms. As such, ASF/SF2 is an important target for cancer therapy, as it is over-expressed in many tumors. Modifications and defects in the alternative splicing pathway are associated with a variety of human diseases. ASF/SF2 is involved in the replication of HIV-1, as HIV-1 needs a delicate balance of spliced and unspliced forms of its viral DNA. ASF/SF2 action in the replication of HIV-1 is a potential target for HIV therapy. ASF/SF2 is also implicated in the production of T cell receptors in

Systemic Lupus Erythematosus Lupus, technically known as systemic lupus erythematosus (SLE), is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body. Symptoms vary among people and may be mild to severe. Comm ...

, altering specific chain expression in T cell receptors through alternative splicing.

Interactions

ASF/SF2 has been shown to

interact Advocates for Informed Choice, dba interACT or interACT Advocates for Intersex Youth, is a 501(c)(3) nonprofit organization using innovative strategies to advocate for the legal and human rights of children with intersex traits. The organizati ...

with: * CDC5L, * CLK1, * PSIP1, *

SFRS2 Splicing factor, arginine/serine-rich 2 is a protein that in humans is encoded by the ''SFRS2'' gene. MDS-associated splicing factor SRSF2 affects the expression of Class III and Class IV isoforms and perturbs granulopoiesis and SRSF2 P95H promote ...

, * SRPK1, *

SRPK2 Serine/threonine-protein kinase SRPK2 is an enzyme that in humans is encoded by the ''SRPK2'' gene. Interactions SRPK2 has been shown to interact with: * ASF/SF2 and * U2AF2 Splicing factor U2AF 65 kDa subunit is a protein that in humans is e ...

, * TOP1, *

U2 small nuclear RNA auxiliary factor 1 Splicing factor U2AF 35 kDa subunit is a protein that in humans is encoded by the ''U2AF1'' gene. Function This gene belongs to the splicing factor SR family of genes . U2AF1 is a subunit of the U2 Auxiliary Factor complex alongside a larger ...

, and *

snRNP70 snRNP70 also known as U1 small nuclear ribonucleoprotein 70 kDa is a protein that in humans is encoded by the ''SNRNP70'' gene. snRNP70 is a small nuclear ribonucleoprotein that associates with U1 spliceosomal RNA, forming the U1snRNP a core comp ...

References

External links

* {{PDBe-KB2, Q07955, Serine/arginine-rich splicing factor 1 Gene expression Human proteins Biology of bipolar disorder