LA/SS-B Human
Description
The LA/SS-B is purified by proprietary chromatographic techniques.
Product Specs
Q&A
What is LA/SS-B and what is its molecular structure?
LA/SS-B is a 47-48 kD ribonucleic protein that functions as an RNA-binding protein primarily residing in the nucleus, though it can also be detected in the cytoplasm. It has a conserved N-terminal domain composed of the La protein-specific domain (La motif), the RNA recognition domain (RRM1), and a highly degenerate RNA recognition domain (RRM2). These domains are linked to a highly-charged C-terminal domain via a 43-amino acid linker . The C-terminal domain contains a putative ATP-binding domain, a phosphorylation site at Ser366, and a nuclear localization signal .
Research has demonstrated that LA/SS-B consists of two protease-resistant domains of 23 and 28 kD, with the latter containing the RNA binding site . The protein is susceptible to proteolysis, resulting in smaller fragments (42kD, 32kD, and 27kD) that remain immunoreactive .
What is the biological function of LA/SS-B?
LA/SS-B serves as a molecular chaperone for RNA, stabilizing the molecule for further processing . It is associated with a spectrum of small RNAs and appears to be involved with RNA polymerase III transcripts, including 5S RNA and tRNAs . The protein binds to UUU-OH located at the 3′-terminus of RNA via an oligouridylate recognition sequence .
The binding capacity of LA/SS-B can be modulated by either the presence of ATP or the phosphorylation of Ser366, both of which negatively affect its ability to bind to polyuridine . This modulation mechanism suggests a regulated role in RNA processing and maturation.
Which autoimmune diseases are associated with anti-LA/SS-B antibodies?
Anti-LA/SS-B antibodies are most commonly associated with two principal autoimmune conditions:
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Sjögren's Syndrome: Anti-LA/SS-B antibodies were originally described as precipitating autoantibodies in sera of Sjögren's Syndrome patients, referred to as SjT . The presence of these autoantibodies has been advocated as a diagnostic aid in Sjögren's Syndrome patients .
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Systemic Lupus Erythematosus (SLE): Anti-LA/SS-B precipitins are commonly found in SLE patients . There appears to be a correlation between anti-LA/SS-B and the absence of nephritis in SLE patients, indicating potential prognostic value .
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Subacute Cutaneous Lupus: Autoantibodies against LA/SS-B are also found in this condition .
What is the relationship between anti-LA/SS-B and anti-Ro/SS-A antibodies?
There is a strong relationship between anti-LA/SS-B and anti-Ro/SS-A antibodies in autoimmune conditions:
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Co-occurrence pattern: Approximately 50% of patients with SLE who have anti-Ro antibody also have anti-La antibody. Similarly, most patients with Sjögren's syndrome who have anti-Ro also have anti-La antibodies .
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Diagnostic significance: It is exceedingly rare to find patients with anti-La antibodies without anti-Ro antibodies when rigorously assessed . This relationship is so consistent that isolated anti-SS-B positivity without anti-SS-A is considered unusual and warrants verification .
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Clinical correlations: Studies have demonstrated that SLE patients with precipitating anti-Ro/SS-A antibodies have a high incidence of serious nephritis (53%), while those with both anti-Ro/SS-A and anti-La/SS-B have a significantly lower frequency of nephritis (9%) . This suggests potential protective effects of combined antibody profiles.
What are the optimal techniques for detecting anti-LA/SS-B antibodies?
Multiple laboratory techniques are available for detecting anti-LA/SS-B antibodies, each with different characteristics:
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Solid-phase immunoassays: These include ELISA, flow fluorimetry, and immunodot assays. These techniques allow for the simultaneous detection of multiple autoantibodies and enhance the sensitivity of autoantibody detection .
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Immunoblotting: Used for confirmation of positive results from other assays .
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Addressable laser beam immunoassay (ALBIA): A newer methodology that offers high throughput and sensitivity .
For optimal accuracy, a multi-stage testing approach is recommended:
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Initial screening using ELISA or ALBIA
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Confirmation of positive results using immunodot or another orthogonal method
Research has shown that confirmation testing significantly improves specificity. In one study, only one-third of sera that tested positive by ELISA or ALBIA were confirmed positive by immunodot .
How can researchers differentiate between true and false positive anti-LA/SS-B results?
Differentiating between true and false positive results requires attention to several factors:
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Antibody titer significance: The mean Antibody Index (AI) titer of sera positive in ALBIA but negative in confirmatory immunodot testing is significantly lower than that of confirmed positive sera (2.297 vs 3.558, p<0.0001) . This suggests that higher thresholds might improve specificity.
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Co-occurring antibodies: Isolated anti-SS-B positivity (without anti-SS-A) is extremely rare, representing only 3.6% of all anti-SS-B positive samples in one large study when rigorously assessed . The absence of co-occurring anti-SS-A should raise suspicion about the validity of the result.
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Clinical correlation: Evaluating the test results in the context of clinical presentation and other laboratory findings is essential for accurate interpretation.
The table below summarizes the characteristics of patients with isolated anti-SS-B antibodies from a large-scale study:
| Variables | Patients, n=61 |
|---|---|
| Female/Male ratio | 43/18 (70%/30%) |
| Age at inclusion, in years (IQR) | 41 (31-59) |
| History of autoimmune disease | 24/61 (39%) |
| Patients with associated antibody | 22/61 (36%) |
| Anti-dsDNA | 5/27 (19%) |
| ENA antibodies | 9/27 (33%) |
| Anti-centromere | 3/9 (33%) |
| Anti-phospholipid | 3/27 (11%) |
| ANCA | 3/27 (11%) |
This data highlights the rarity and specific characteristics of truly isolated anti-SS-B positive cases .
How should LA/SS-B be used in experimental models of autoimmunity?
LA/SS-B can be effectively utilized in several experimental approaches for autoimmunity research:
What approaches can be used to study the structural domains of LA/SS-B?
Studying LA/SS-B structural domains requires specialized methodologies:
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Protease digestion studies: LA/SS-B can be divided into its constituent domains through controlled protease digestion, yielding the 23 kD and 28 kD fragments that retain different functional and immunological properties .
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Recombinant protein expression: The individual domains of LA/SS-B can be expressed as recombinant proteins for structural and functional studies. The RNA binding domain (28 kD) is particularly important as it contains the RNA binding site and is the target of most autoantibodies .
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Immunoprecipitation techniques: Monoclonal antibodies such as A1, A2, and A3 are capable of immunoprecipitating the 48-kD SS-B protein along with its associated RNAs . This allows for studies of RNA-protein interactions under various conditions.
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Site-directed mutagenesis: Modifying key residues, particularly at the phosphorylation site (Ser366), can help understand how post-translational modifications affect LA/SS-B function and immunogenicity .
How does LA/SS-B evolutionary conservation impact autoantibody recognition?
The evolutionary conservation of LA/SS-B has profound implications for autoimmunity research:
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Cross-species reactivity: Human autoantibodies targeting LA/SS-B react with the protein from various mammalian species, indicating that the autoepitopes are highly conserved . This conservation suggests these epitopes may be functionally critical regions of the molecule.
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Differential recognition patterns: While human autoantibodies recognize conserved epitopes across species, murine monoclonal antibodies show species restrictions. They react with human, monkey, bovine, and rabbit LA/SS-B but not with rat or mouse LA/SS-B . This differential recognition provides insights into the nature of autoepitopes.
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Functional significance: Human autoantibodies appear to target epitopes that may represent active or catalytic sites of molecules serving important cellular functions . This targeting of functional regions may explain the pathogenicity of these autoantibodies.
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Experimental implications: The conservation patterns must be considered when selecting animal models for autoimmunity research. The species-specific differences in LA/SS-B recognition can affect the translatability of findings between models and human disease .
What is the significance of LA/SS-B post-translational modifications in disease pathogenesis?
Post-translational modifications of LA/SS-B play potentially crucial roles in disease pathogenesis:
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Phosphorylation effects: The phosphorylation of Ser366 negatively modulates the binding of LA/SS-B to polyuridine . This regulation mechanism may be disrupted in disease states, affecting RNA processing and potentially contributing to autoimmunity.
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Cellular localization changes: LA/SS-B can relocate from its primary nuclear position to the cytoplasm or cell membrane under certain conditions such as UV irradiation, viral infection, or serum starvation . These translocation events may expose normally sequestered epitopes to the immune system.
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Proteolytic processing: LA/SS-B is susceptible to proteolysis, resulting in smaller fragments that remain immunoreactive . The pattern of fragmentation may differ in disease states, potentially generating novel epitopes or exposing cryptic ones.
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Research approaches: Studying these modifications requires sophisticated techniques such as mass spectrometry, phospho-specific antibodies, subcellular fractionation, and functional assays comparing native and modified forms of LA/SS-B.
Product Science Overview
LA/SS-B, also known as Sjögren syndrome type B antigen, is a human recombinant protein frequently associated with autoimmune diseases such as systemic lupus erythematosus (SLE) and Sjögren’s syndrome . This protein is a 47 kDa polypeptide that plays a crucial role in RNA metabolism and is often targeted by autoantibodies in patients with these conditions .
The LA/SS-B protein is involved in various aspects of RNA metabolism, including:
- Binding and protecting 3-prime UUU (OH) elements of newly transcribed RNA by RNA polymerase III .
- Processing 5-prime and 3-prime ends of pre-tRNA precursors .
- Acting as an RNA chaperone and binding viral RNAs linked to hepatitis C virus .
In the nucleus, LA/SS-B facilitates the production of tRNAs by acting as an RNA polymerase III transcription factor. It attaches to the U-rich 3’UTR of nascent transcripts, aiding in their folding and maturation . In the cytoplasm, LA/SS-B facilitates the translation of specific mRNAs by acting as a translation factor .
LA/SS-B is a highly conserved protein across species and is readily disposed to proteolysis, resulting in smaller immunoreactive polypeptides . It is a nuclear protein of 48 kilodaltons with two structural domains generated by proteolytic cleavage . The human La (SS-B) autoantigen is an abundantly expressed putative RNA chaperone, functioning in various intracellular processes involving RNA .
Anti-La/SS-B autoantibodies were originally found as precipitating autoantibodies in the sera of Sjögren’s syndrome patients and are most frequently found in Sjögren’s syndrome, SLE, and subacute cutaneous lupus . There is also a correlation between anti-La/SS-B and the absence of nephritis in SLE patients .
Recombinant LA/SS-B is produced in various systems, including E. coli and SF9 insect cells . The recombinant protein is typically a single, non-glycosylated polypeptide chain with a molecular mass of approximately 47.6 kDa . It is purified using proprietary chromatographic techniques and is available in different formulations for laboratory research use .
