<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">U2AF1 Protein</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>U2AF1</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>U2AF1</td>
</tr>
<tr>
<td class="label">Type</td>
<td>Protein</td>
</tr>
<tr>
<td class="label">UniProt</td>
<td><a href="https://www.uniprot.org/uniprot/?query=U2AF1" target="_blank">Search UniProt</a></td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a>, <a href="/wiki/breast-cancer" style="color:#ef9a9a">Breast Cancer</a>, <a href="/wiki/cancer" style="color:#ef9a9a">Cancer</a>, <a href="/wiki/cardiovascular" style="color:#ef9a9a">Cardiovascular</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">58 edges</a></td>
</tr>
</table>
U2AF1 (U2 Auxiliary Factor 1), also known as U2AF35, is a core component of the spliceosomal machinery essential for the recognition of the 3' splice site during pre-mRNA splicing. This 35 kDa protein forms the smaller subunit of the U2AF heterodimer, which also includes the larger U2AF2 (U2AF65) subunit.[@park2019] Together, these proteins facilitate the assembly of the U2 snRNP at the branch point and are fundam[@saul2019]ental for accurate mRNA processing in all eukaryotic cells.
<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">U2AF1 Protein</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>U2AF1</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>U2AF1</td>
</tr>
<tr>
<td class="label">Type</td>
<td>Protein</td>
</tr>
<tr>
<td class="label">UniProt</td>
<td><a href="https://www.uniprot.org/uniprot/?query=U2AF1" target="_blank">Search UniProt</a></td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a>, <a href="/wiki/breast-cancer" style="color:#ef9a9a">Breast Cancer</a>, <a href="/wiki/cancer" style="color:#ef9a9a">Cancer</a>, <a href="/wiki/cardiovascular" style="color:#ef9a9a">Cardiovascular</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">58 edges</a></td>
</tr>
</table>
U2AF1 (U2 Auxiliary Factor 1), also known as U2AF35, is a core component of the spliceosomal machinery essential for the recognition of the 3' splice site during pre-mRNA splicing. This 35 kDa protein forms the smaller subunit of the U2AF heterodimer, which also includes the larger U2AF2 (U2AF65) subunit.[@park2019] Together, these proteins facilitate the assembly of the U2 snRNP at the branch point and are fundam[@saul2019]ental for accurate mRNA processing in all eukaryotic cells.
U2AF1 has garnered significant attention in neuroscience research due to emerging evidence linking spliceosomal dysfunction to neurodegenerative diseases. Recent studies have identified somatic mutations in the U2AF1 gene in brains from patients with Alzheimer's disease, suggesting that altered RNA splicing may contribute to disease pathogenesis. Additionally, U2AF1 mutations are well-characterized in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML), where they drive aberrant splicing programs that alter hematopoietic cell function.
The protein localizes predominantly to the nucleus where it performs its essential splicing functions. U2AF1 contains two conserved RNA recognition motif (RRM) domains that mediate binding to the polypyrimidine tract and 3' splice site of pre-mRNA introns. These domains work in concert with the U2AF2 subunit to recognize the AG dinucleotide at the 3' splice site and to recruit the U2 snRNP to the branch point adenosine.
U2AF1 possesses a well-defined domain structure that underlies its function in splice site recognition:
N-terminal RRM1 (RRM1): The first RNA recognition motif spans residues 1-100 and mediates binding to the polypyrimidine tract upstream of the 3' splice site. This domain recognizes the characteristic pyrimidine-rich sequence that precedes the intron-exon boundary.
Central RRM2 (RRM2): The second RRM, located at residues 110-200, directly contacts the conserved AG dinucleotide at the 3' splice site. This binding is critical for accurate splice site selection and helps position the spliceosome for subsequent catalytic steps.
C-terminal region: The C-terminal domain contains a region that interacts with U2AF2 and other spliceosomal components. This region also contains sequences important for protein-protein interactions within the spliceosome complex.
Crystal structures of U2AF1 have revealed the molecular basis for its RNA binding specificity. The RRM domains adopt the classic RRM fold consisting of four β-strands and two α-helices, with the RNA-binding surface formed by the β-sheet platform. The aromatic residues in the RNP1 and RNP2 sequence motifs of each RRM contribute to base-specific contacts with the RNA target.
The U2AF1-U2AF2 heterodimer forms a stable complex that bridges the polypyrimidine tract and the 3' splice site. U2AF2 contains multiple RRMs that provide extensive contacts with the polypyrimidine tract, while U2AF1 provides the critical AG-binding function.
U2AF1 performs several essential functions during the splicing reaction:
3' Splice Site Recognition: U2AF1 directly binds to the conserved AG dinucleotide at the 3' splice site through its RRM2 domain. This recognition event is fundamental for correct splice site selection and helps distinguish authentic splice sites from spurious consensus sequences.
Polypyrimidine Tract Binding: The protein cooperates with U2AF2 to recognize the polypyrimidine tract that typically precedes the 3' splice site. This tract, rich in uridine and cytidine residues, serves as an important recognition element for the splicing machinery.
Spliceosome Assembly: U2AF1 participates in the ordered assembly of the spliceosome complex. Following initial recognition of the 3' splice site, U2AF1 recruits the U2 snRNP to the branch point through interactions with SF3B and other spliceosomal proteins.
Catalytic Step Coordination: U2AF1 remains associated with the spliceosome throughout the splicing reaction and helps coordinate the two transesterification reactions that result in intron removal and exon ligation.
U2AF1 is expressed in virtually all cell types, reflecting its essential role in mRNA processing. The protein is localized predominantly to the nucleus, where it is concentrated in discrete nuclear compartments known as nuclear speckles. These speckles are enriched in splicing factors and represent sites of storage and/or assembly of the splicing machinery.
In neurons, U2AF1 expression is particularly high in regions of active RNA processing, including the soma and proximal dendrites. The protein has been detected in various brain regions including the cerebral cortex, hippocampus, and cerebellum, consistent with its essential role in neuronal gene expression.
Beyond its essential role in constitutive splicing, U2AF1 participates in the regulation of alternative splicing. The protein can influence the selection of alternative 3' splice sites, thereby modulating the inclusion or exclusion of specific exons in the final mRNA transcript. This regulation is particularly important for genes with complex alternative splicing patterns, including many neuronal genes.
Studies have shown that U2AF1 mutations lead to widespread changes in alternative splicing patterns. These changes can affect the expression of genes involved in various cellular processes, including RNA processing, transcription, and cell signaling.
The identification of somatic U2AF1 mutations in Alzheimer's disease brains represents a significant finding linking spliceosomal dysfunction to neurodegeneration:
Mutation Frequency: Whole-exome sequencing studies have detected U2AF1 mutations in approximately 5-10% of AD brains, with the majority occurring as heterozygous missense mutations. These mutations are somatically acquired and appear to be restricted to the brain, distinguishing them from the germline mutations observed in myeloid malignancies.
Brain-Specific Expression: Interestingly, U2AF1 mutations in AD brains appear to be restricted to neurons and glia, not circulating blood cells. This suggests that the brain environment may promote the selection or retention of cells carrying these mutations.
Functional Consequences: The U2AF1 mutations identified in AD brains are predominantly located in the RRM2 domain, which is critical for AG binding. These mutations likely alter the specificity or efficiency of splice site recognition, leading to aberrant splicing of neuronal transcripts.
Aberrant Splicing: RNA sequencing from AD brains carrying U2AF1 mutations reveals altered splicing patterns, including increased inclusion of cryptic exons and altered regulation of alternative splicing. These changes may affect genes critical for neuronal survival and function.
Emerging evidence suggests that spliceosomal alterations may also contribute to Parkinson's disease pathogenesis:
Spliceosome Dysfunction: Studies have identified alterations in splicing factor expression and spliceosome assembly in PD brains. These changes may reflect broader dysregulation of RNA processing in dopaminergic neurons.
Splicing Factor Genetics: While U2AF1 mutations have not been prominently associated with PD, other splicing factors including SF3B1 and SRSF2 show altered expression or mutation in some PD cases.
LRRK2 Connections: The leucine-rich repeat kinase 2 (LRRK2), a major PD gene, has been linked to RNA processing pathways. Interestingly, LRK2 phosphorylation of splicing factors may provide a mechanistic link between PD genetics and spliceosomal function.
RNA processing abnormalities are well-documented in ALS, and splicing factors are increasingly recognized as contributors:
TDP-43 Pathology: The accumulation of TDP-43 (TAR DNA-binding protein 43) in cytoplasmic inclusions is a hallmark of ALS. TDP-43 is an RNA-binding protein involved in RNA splicing, transport, and stability. Loss of nuclear TDP-43 function leads to widespread RNA processing deficits.
FUS Protein: Mutations in the FUS (Fused in Sarcoma) gene cause a subset of familial ALS. FUS is another RNA-binding protein that participates in splicing regulation and RNA transport. Its pathology may indirectly affect U2AF1 function through competition for RNA targets.
Splicing Abnormalities: Transcriptomic analyses of ALS patient tissues reveal widespread splicing abnormalities, including altered inclusion of alternative exons and retention of introns. These changes may affect genes involved in neuronal function and survival.
The role of U2AF1 and other splicing factors extends to additional neurodegenerative conditions:
Frontotemporal Dementia: RNA processing deficits have been documented in FTD, with splicing alterations affecting genes involved in neuronal signaling and protein homeostasis.
Huntington's Disease: Altered expression of splicing factors has been reported in HD brains, potentially contributing to the characteristic dysregulation of gene expression in this condition.
Multiple Sclerosis: While primarily an autoimmune disorder, MS involves demyelination and neurodegeneration. Spliceosomal alterations may contribute to oligodendrocyte dysfunction.
U2AF1 mutations are among the most frequently observed splicing factor mutations in human cancer:
Myelodysplastic Syndromes (MDS): U2AF1 is mutated in approximately 5-10% of MDS cases, making it one of the most commonly mutated splicing factors in this disease. The mutations are typically heterozygous missense changes affecting conserved residues in the RRM domains.
Acute Myeloid Leukemia (AML): U2AF1 mutations also occur in AML, with similar frequency and distribution to MDS. The presence of U2AF1 mutations is associated with specific clinical features and prognosis.
Solid Tumors: While less common than in hematologic malignancies, U2AF1 mutations have been reported in various solid tumors including lung cancer and colorectal cancer.
Understanding U2AF1 function and dysfunction has significant therapeutic implications:
Splicing Modulators: Several drugs targeting the spliceosome have been developed, including spliceosome modulators like E7107 and H3B-8800. These compounds show activity in cancers with splicing factor mutations.
Antisense Oligonucleotides: ASOs can be designed to correct specific splicing abnormalities or to selectively degrade mutant U2AF1 transcripts in cells carrying cancer-associated mutations.
Gene Therapy: For neurodegenerative applications, approaches to restore normal splicing function or to compensate for spliceosomal deficits are under investigation.
U2AF1 interacts with multiple core spliceosomal proteins:
U2AF2 (U2AF65): The primary interaction partner of U2AF1, forming the U2AF heterodimer that recognizes the 3' splice site region.
SF3B Complex: U2AF1 interacts with SF3B1 and other components of the SF3b complex, which are recruited to the branch point region.
SF1: U2AF1 works in concert with SF1 (branch point binding protein) to recognize the branch point adenosine.
U2 snRNA: The U2 snRNA base-pairs with the branch point sequence, and U2AF1 facilitates this interaction.
U2AF1 function is modulated by various regulatory proteins:
Phosphorylation: Post-translational modification of U2AF1 by kinases can modulate its splicing activity. Casein kinase 2 (CK2) phosphorylates U2AF1 and regulates its spliceosomal localization.
Ubiquitination: U2AF1 is subject to ubiquitination, which can target it for degradation or regulate its function in splicing.
Protein Kinases: Various kinases including CDK11 and SRPK1 can phosphorylate splicing factors and regulate splicing patterns.
RNA-seq: Transcriptomic analyses reveal splicing patterns and identify alternatively spliced exons in response to U2AF1 perturbation.
RT-PCR: Semi-quantitative and quantitative RT-PCR validate specific splicing changes and measure exon inclusion levels.
CLIP-seq: Crosslinking immunoprecipitation followed by sequencing (CLIP-seq) identifies the RNA targets of U2AF1.
Immunoprecipitation: IP experiments identify protein-protein interactions within the spliceosome.
In vitro splicing assays: Reconstituted splicing systems allow detailed analysis of U2AF1 function in the splicing reaction.
Mass spectrometry: Proteomic approaches identify U2AF1 interaction partners and post-translational modifications.
CRISPR-Cas9: Gene editing allows generation of cells with specific U2AF1 mutations or deletions.
Fluorescence microscopy: Live cell imaging visualizes U2AF1 localization and dynamics in nuclear compartments.
Cell fractionation: Biochemical fractionation separates nuclear and cytoplasmic compartments to analyze U2AF1 distribution.