PRP18 Antibody

What is PRP18 and what role does it play in pre-mRNA splicing?

PRP18 is an evolutionarily conserved splicing factor specifically required for the second step of pre-mRNA splicing. In human cells, hPrp18 shares approximately 30% sequence identity with its yeast counterpart, with stronger conservation in specific domains .

The primary function of PRP18 is facilitating the second catalytic step of splicing, where it enables the transesterification reaction that joins two exons while releasing the intron lariat. Immunodepletion studies in both yeast and human systems have definitively demonstrated that when PRP18 is removed from cell extracts, the second step of splicing is severely inhibited or completely abolished .

Recent research has revealed a crucial role for PRP18 in 3'-splice site (3'SS) selection and fidelity. Studies in S. cerevisiae show that absence of Prp18p results in genome-wide activation of alternative 3'SS, including highly unusual non-YAG sequences . This finding indicates that PRP18 functions as a critical component for ensuring proper recognition of consensus 3'SS sequences, which is essential for accurate gene expression.

At the molecular level, PRP18 demonstrates a specific temporal association with the spliceosome, interacting with it predominantly during the second step of splicing . This transient but crucial association highlights PRP18's specialized role in the splicing machinery.

How can PRP18 antibodies be used to study splicing mechanisms?

PRP18 antibodies have proven to be valuable tools for investigating pre-mRNA splicing mechanisms through several experimental approaches:

Immunodepletion Experiments

One of the most informative techniques involves using anti-PRP18 antibodies to deplete the protein from cell extracts. Multiple studies have demonstrated that extracts immunodepleted of PRP18 show significant inhibition of the second step of splicing . The specificity of this effect can be confirmed through restoration of splicing activity by adding recombinant PRP18 protein.

A typical immunodepletion protocol includes:

• Incubating cell extracts with protein A-bound αPRP18 antibodies

• Removing released antibodies with subsequent protein A-Sepharose incubation

• Confirming depletion efficiency via Western blot analysis

• Assessing splicing activity through in vitro splicing assays

Western Blotting for Depletion Confirmation

Western blotting is essential for confirming PRP18 depletion efficiency, as shown in this representative analysis from yeast extract studies:

This quantitative assessment is critical for interpreting subsequent functional data .

Tracking Spliceosome Assembly and Function

PRP18 antibodies can be used to monitor the association of PRP18 with the spliceosome during different stages of splicing. Research has shown that hPrp18 is "bound tightly to the spliceosome only during the second step of splicing" , making antibodies useful for precise temporal analysis of spliceosomal dynamics.

Studying Protein-Protein Interactions

Antibodies facilitate co-immunoprecipitation experiments to identify PRP18's interaction partners within the spliceosome. Studies have revealed that PRP18's role in 3'SS fidelity is facilitated by interactions with Slu7p and Prp8p , information obtained partly through antibody-based interaction studies.

Advanced Application: Splicing Chase Experiments

For advanced mechanistic investigations, PRP18 antibodies have been employed in splicing chase experiments to demonstrate that the second step of splicing after immunodepletion "does not require ATP, providing strong support for a transesterification mechanism for the second step of pre-mRNA splicing" . This represents a sophisticated application for addressing fundamental mechanistic questions about splicing catalysis.

What are the differences between PRP18 homologs across species?

PRP18 demonstrates evolutionary conservation across species while exhibiting notable differences that reveal both functional conservation and specialization:

Association with snRNPs

A key difference between species involves association with small nuclear ribonucleoproteins (snRNPs):

Fission Yeast Prp18

The fission yeast (Schizosaccharomyces pombe) Prp18 provides an interesting intermediate case. Studies using plasmid complementation showed varying degrees of functionality:

This data reveals complete loss of complementation with the triple alanine mutant, highlighting critical residues for function .

Novel Functions Across Species

Beyond its conserved role in splicing, species-specific functions have been discovered:

• In influenza virus-infected cells, Prp18 stimulates viral RNA synthesis through interaction with the viral nucleoprotein (NP)

• In S. cerevisiae, Prp18p prevents activation of alternative 3'SS, including unusual non-YAG sequences

These differences demonstrate how a core splicing factor has evolved additional functions in different organisms while maintaining its fundamental role in the second step of splicing.

How does PRP18 interact with other splicing factors?

PRP18 functions within a complex network of splicing factors, with interactions that are essential for its role in the second step of pre-mRNA splicing:

Key Interaction Partners

Research has identified several important protein-protein interactions involving PRP18:

• Slu7p and Prp8p: Recent studies show that "the role of Prp18p in 3′SS fidelity is facilitated by interactions with Slu7p and Prp8p" . This interaction network is crucial for recognizing the correct 3' splice site.

• Prp22p Helicase: The fidelity function of Prp18p is "synergized by the downstream proofreading activity of the Prp22p helicase" , suggesting a cooperative mechanism to ensure accurate splicing.

• NP (Nucleoprotein) in Viral Systems: In the context of influenza virus infection, "The interaction between NP and Prp18 is responsible for stimulation of viral RNA synthesis by Prp18" . This demonstrates how PRP18's interactions can be coopted in viral systems.

Temporal Dynamics of Interactions

The interactions of PRP18 with the spliceosome are highly regulated temporally. Studies have shown that "hPrp18 is bound tightly to the spliceosome only during the second step of splicing" , highlighting the transient nature of these interactions.

Functional Significance of Interactions

The interaction with Slu7p is particularly notable. While Prp18p and Slu7p work together to ensure 3'SS fidelity, research has shown that the role of Prp18p "cannot be fulfilled by Slu7p, identifying a unique role for Prp18p in 3′SS fidelity" .

Structural Basis of Interactions

While the complete structural details of PRP18's interactions remain to be fully characterized, the conserved 90-amino-acid region near the carboxyl terminus appears critical for many of these interactions, as it "may define a new protein motif" involved in spliceosomal protein-protein interactions.

Advanced Perspective: Spliceosome Remodeling

For researchers studying the dynamic assembly and remodeling of the spliceosome, it's important to note that PRP18 is part of a "network of spliceosomal interactions...required to promote the fidelity of the recognition of consensus 3′SS sequences" . This suggests that PRP18 contributes to the structural changes that occur during the transition from the first to second catalytic step of splicing.

What experimental techniques are most effective for studying PRP18 function?

Multiple experimental approaches have proven valuable for investigating PRP18's functions, with varying levels of complexity and informational yield:

Basic Techniques

• Immunoblotting/Western Blot Analysis:

  • Essential for detecting PRP18 protein levels and modifications

  • Can reveal post-translational modifications, as seen in fission yeast where "immunoblotting analyses of wild-type SpPrp18 protein reveals an additional slower migrating species with ∼3–4-kDa increased size"

  • Useful for confirming depletion efficiency in immunodepletion experiments

• Plasmid Complementation Assays:

  • Assess functionality of wild-type or mutant PRP18 constructs

  • Example results from fission yeast studies:

  Strain	Growth without thiamine	Growth with thiamine
  Wild-type	Robust	Poor
  prp18-5 mutant	Slow	Inviable
  + rescue plasmid	Rescued	Rescued

  This approach clearly demonstrated that "wild-type protein expressed from the leu1 locus fully complemented the spprp18Δ null allele, but the SpPrp18-5 protein only partially supported growth" .

Advanced Techniques

• In Vitro Splicing Assays:

  • The gold standard for directly assessing PRP18's role in splicing

  • Typically uses radioactively labeled pre-mRNA substrates followed by gel analysis

  • Quantitative analysis can determine kinetics of each splicing step in the presence or absence of PRP18

  • Studies show that in PRP18-depleted extracts, "the second step of splicing is inhibited...The 2/3 lariat and exon 1 intermediates accumulate"

• Immunodepletion-Complementation:

  • Combines antibody-mediated depletion followed by add-back of recombinant protein

  • Particularly powerful for establishing direct causality

  • "In HeLa cell extracts immunodepleted of hPrp18, the second step of pre-mRNA splicing is abolished. Splicing activity is restored by the addition of recombinant hPrp18"

• RNA-Seq for Splicing Fidelity Analysis:

  • Genome-wide approach to identify alternative splicing events

  • Revealed that "absence of the Prp18p splicing factor results in genome-wide activation of alternative 3′SS in S. cerevisiae, including highly unusual non-YAG sequences"

• Structure-Function Analysis:

  • Systematic mutagenesis to identify critical residues/domains

  • Particularly informative for the conserved C-terminal domain

  • Studies with triple alanine mutants (G196A/V197A/T198A) demonstrated complete loss of function

• Protein-RNA Crosslinking:

  • For identifying direct interactions between PRP18 and RNA components

  • Studies suggest PRP18 may interact with RNA during splicing, particularly in viral RNA synthesis where "Prp18 dissociates newly synthesized RNA from the template after the early elongation step"

• Co-immunoprecipitation:

  • Essential for mapping the interaction network of PRP18

  • Has identified interactions with factors like Slu7p, Prp8p, and Prp22p

For comprehensive analysis of PRP18 function, combining these approaches provides complementary insights into both molecular mechanisms and biological significance.

How do PRP18 mutations affect splicing fidelity?

PRP18 mutations have profound effects on splicing fidelity, particularly at the 3' splice site selection stage, with implications for gene expression accuracy:

Impact on 3' Splice Site Selection

Recent research has revealed that "absence of the Prp18p splicing factor results in genome-wide activation of alternative 3′SS in S. cerevisiae, including highly unusual non-YAG sequences" . This indicates that PRP18 normally functions to restrict splicing to consensus 3'SS sequences.

Molecular Basis of Altered Fidelity

The mechanism underlying this decreased fidelity involves several factors:

• Aberrant Splice Site Recognition: In the absence of Prp18p, usage of non-canonical 3'SS "is enhanced by upstream poly(U) tracts and by their potential to interact with the first intronic nucleoside" . This allows improper sites to "dock in the spliceosome active site instead of the normal 3′SS" .

• Loss of Proofreading Coordination: PRP18's fidelity function works in concert with the Prp22p helicase. Studies show this fidelity function "is synergized by the downstream proofreading activity of the Prp22p helicase, but is independent from another late splicing helicase, Prp43p" . When PRP18 is mutated, this coordination is disrupted.

Specific Mutations with Known Effects

In fission yeast, specific mutations in the PRP18 gene produce distinct phenotypes:

Structural Implications

The triple alanine mutation data suggests that residues G196, V197, and T198 are critical for PRP18 function, likely affecting proper protein folding or interaction with other spliceosomal components. This region may be part of the conserved C-terminal domain that is "strongly homologous" across species .

Evolutionary Conservation of Fidelity Function

The role of PRP18 in maintaining splicing fidelity appears to be evolutionarily conserved. While the exact mechanisms may differ between species, the fundamental function in ensuring accurate 3'SS recognition is preserved, suggesting its critical importance for proper gene expression.

Advanced Research Perspective

For researchers studying splicing fidelity mechanisms, it's important to note that "spliceosomes exhibit remarkably relaxed 3′SS sequence usage in the absence of Prp18p" . This suggests that PRP18 is a key component of the quality control mechanisms that restrict spliceosomal activity to appropriate splice sites, preventing potentially deleterious mis-splicing events.

What challenges exist in generating specific antibodies against PRP18?

Generating specific and effective antibodies against PRP18 presents several technical challenges that researchers should consider:

Low Endogenous Expression Levels

PRP18, like many splicing factors, is typically expressed at low levels in cells. As noted in the literature, "PRP proteins are typically difficult to detect" . This means that generating an immune response against the native protein can be challenging, often necessitating the use of recombinant protein as an immunogen.

Cross-Reactivity Concerns

The search results indicate potential cross-reactivity issues with antibodies against PRP18. For example, "a 55-kD protein that cross-reacts with antibodies against hPrp18 is a constituent of the U4/U6 and U4/U6 x U5 snRNP particles" . This suggests that antibodies may recognize related proteins, requiring careful validation.

Protein Modification Status

Post-translational modifications can affect antibody recognition. Studies in fission yeast revealed "an additional slower migrating species with ∼3–4-kDa increased size" of the PRP18 protein, which could complicate antibody development and application if the epitopes are masked or altered by these modifications.

Species-Specific Considerations

Since PRP18 shows only moderate sequence conservation between species (approximately 30% identity between human and yeast versions ), antibodies developed against one species' PRP18 may not recognize orthologs from other species. Researchers should carefully consider the target organism when selecting or developing antibodies.

Validation Strategies

To address these challenges, multiple validation approaches are recommended:

• Western Blot Analysis:

  • Include positive controls (recombinant PRP18)

  • Include negative controls (extracts from PRP18-disrupted strains)

  • Test for cross-reactivity with related proteins

• Immunodepletion Efficiency:

  • Verify depletion through both Western blotting and functional assays

  • Ensure that "quantitation of the blot pictured showed that >95% of the PRP18 had been removed"

• Functional Validation:

  • Confirm that immunodepletion results in expected functional defects in splicing assays

  • Verify that adding back recombinant protein restores activity

Advanced Consideration: Epitope Selection

For researchers developing new antibodies against PRP18, epitope selection is critical. The conserved 90-amino-acid region near the C-terminus may provide epitopes that function across species, while more variable regions may offer greater species specificity but potentially reduced cross-species utility.

How can immunodepletion experiments with PRP18 antibodies be optimized?

Immunodepletion is a powerful technique for studying PRP18 function, but requires careful optimization to achieve reliable and interpretable results:

Protocol Optimization

Based on published methodologies, an effective immunodepletion protocol for PRP18 typically includes:

• Antibody Preparation:

  • Use affinity-purified antibodies for greater specificity

  • Pre-clear antibodies to remove non-specific components

• Depletion Process:

  • "Immunodepletion of a yeast extract was carried out by incubating extracts with protein A-bound αPRP18"

  • Multiple rounds of depletion may be necessary for maximum removal

  • "Some antibody was released from the resin during this incubation, and most of the released antibody was removed by a subsequent incubation with protein A-Sepharose"

• Validation of Depletion Efficiency:

  • Western blot analysis to confirm depletion (>95% removal is achievable )

  • Include appropriate controls (preimmune serum-treated extracts, untreated extracts)

Extract Preparation Considerations

The starting material quality significantly impacts immunodepletion success:

Functional Validation

Confirmation that the observed effects are specifically due to PRP18 depletion is critical:

• Compare with Control Depletions:

  • "The kinetics of splicing in the preimmune antibody-depleted extract and an untreated extract were essentially identical"

  • This confirms specificity of the observed effects

• Complementation Testing:

  • Add back recombinant PRP18 to restore activity

  • "Splicing activity is restored by the addition of recombinant hPrp18, demonstrating that hPrp18 is required for the second step"

Advanced Experimental Design

For researchers conducting detailed mechanistic studies:

• Time Course Analysis:

  • Examining the kinetics of splicing intermediates accumulation provides insight into the specific stage affected

  • "In Figure 2B the kinetics of disappearance of pre-mRNA in the two extracts are compared; in Figure 2, C and D, the kinetics of formation of 2/3 lariat, exon 1, and mRNA are shown"

• Partial Depletion Studies:

  • Titrating the degree of PRP18 depletion can reveal threshold effects

  • Results showing "the second step of splicing is not completely inhibited, and mRNA is produced slowly" suggest either incomplete depletion or non-absolute requirement

• Combined Depletion Studies:

  • Simultaneously depleting PRP18 and interacting factors (e.g., Slu7p, Prp22p)

  • This approach can reveal synergistic effects and functional redundancies

These optimized approaches ensure that immunodepletion experiments with PRP18 antibodies yield reliable and interpretable results for investigating splicing mechanisms.

What are the key controls needed when using PRP18 antibodies in research?

When using PRP18 antibodies in research, implementing appropriate controls is essential for ensuring reliable and interpretable results:

Essential Controls for Western Blotting

• Positive Controls:

  • Recombinant PRP18 protein (to confirm antibody reactivity)

  • Extracts from wild-type cells known to express PRP18

• Negative Controls:

  • Extracts from PRP18 knockout/disrupted strains

  • As demonstrated in published work: "from an extract of the strain DH120R-1(2-2), in which the PRP18 gene had been disrupted"

• Loading Controls:

  • Probing for housekeeping proteins to ensure equal loading

  • Particularly important when comparing PRP18 levels across conditions

• Antibody Specificity Controls:

  • Preimmune serum (from the same animal before immunization)

  • Secondary antibody only (to detect non-specific binding)

Controls for Immunodepletion Experiments

• Mock Depletion Controls:

  • Preimmune antibody-depleted extracts: "The kinetics of splicing in the preimmune antibody-depleted extract and an untreated extract were essentially identical"

  • Beads-only treatment (to control for non-specific adsorption)

• Partial Depletion Assessment:

  • Quantitative Western blot to determine depletion efficiency

  • "Quantitation of the blot pictured showed that >95% of the PRP18 had been removed"

• Functional Rescue Controls:

  • Add-back of recombinant PRP18 to depleted extracts

  • "Splicing activity is restored by the addition of recombinant hPrp18, demonstrating that hPrp18 is required for the second step"

Controls for Immunoprecipitation Studies

• Input Sample:

  • Always analyze an aliquot of the starting material

  • Allows calculation of enrichment/depletion factors

• Non-specific Binding Controls:

  • IgG or preimmune serum immunoprecipitation

  • Beads-only treatment

• Validation of Interacting Partners:

  • Reciprocal co-immunoprecipitation with antibodies against suspected interaction partners

  • Important for confirming interactions with factors like "Slu7p and Prp8p"

Advanced Experimental Controls

• Cross-Species Complementation Controls:

  • Using yeast PRP18 to complement human cells and vice versa

  • "Splicing activity can be restored to hPrp18-depleted HeLa cell extracts by yeast Prp18"

• Temperature Sensitivity Controls:

  • For temperature-sensitive mutants, include permissive and non-permissive conditions

  • Particularly relevant for yeast studies with conditional alleles

• Mutant Protein Controls:

  • Include non-functional mutants such as the triple alanine mutation (G196A/V197A/T198A)

  • Provides negative controls for complementation experiments

How does PRP18 contribute to the structural integrity of the spliceosome?

PRP18 plays a specialized role in maintaining the structural integrity of the spliceosome, particularly during the transition to and execution of the second catalytic step of splicing:

Temporal Association with the Spliceosome

Unlike many core spliceosomal proteins, PRP18 exhibits a highly specific temporal association. Research shows that "hPrp18 is bound tightly to the spliceosome only during the second step of splicing" . This transient association suggests a role in stabilizing or remodeling the spliceosome specifically during this critical phase.

Regulation of Spliceosomal Conformational Changes

PRP18 appears to facilitate the structural transitions necessary for the second transesterification reaction. Studies in yeast extracts showed that after PRP18 depletion, the second step of splicing "does not require ATP, providing strong support for a transesterification mechanism for the second step of pre-mRNA splicing, and suggesting that PRP18 acts during the second transesterification reaction" . This indicates PRP18 may help position the reactive groups for catalysis.

3' Splice Site Recognition Complex

PRP18 forms part of a network that ensures accurate 3'SS recognition. Research has identified "a network of spliceosomal interactions centered on Prp18p which are required to promote the fidelity of the recognition of consensus 3′SS sequences" . This network includes interactions with Slu7p and Prp8p, suggesting PRP18 helps organize the 3' splice site recognition complex.

Ensuring Proper RNA-Protein Interactions

In the context of viral RNA synthesis, PRP18 has been shown to function "as a chaperone for NP to facilitate the formation of NP-RNA complexes" . While this is in a viral context, it suggests a potential role for PRP18 in facilitating proper RNA-protein interactions within the spliceosome as well.

Prevention of Inappropriate Splice Site Usage

A critical function of PRP18 appears to be preventing the use of non-canonical splice sites. Without PRP18, "spliceosomes exhibit remarkably relaxed 3′SS sequence usage...including highly unusual non-YAG sequences" . This indicates that PRP18 contributes to a structural conformation of the spliceosome that restricts catalysis to appropriate splice sites.

Advanced Structural Considerations

For researchers focused on spliceosome structure:

• Active Site Architecture: PRP18 likely influences the architecture of the spliceosomal active site, particularly in positioning the 3' splice site relative to the 5' splice site and the branched intermediate.

• Coordination with Helicases: The finding that PRP18's fidelity function "is synergized by the downstream proofreading activity of the Prp22p helicase" suggests coordination between PRP18-mediated structural organization and helicase-mediated conformational changes.

• Evolutionary Implications: The fact that a 90-amino-acid region near the carboxyl terminus of hPrp18 is "strongly homologous to yeast Prp18 and is also conserved in rice and nematodes" suggests this domain mediates evolutionarily conserved structural interactions within the spliceosome.

Understanding PRP18's contribution to spliceosomal structure provides insight into how this complex macromolecular machine achieves the precision required for accurate pre-mRNA processing.