Recombinant Human Putative ATP-binding cassette sub-family C member 13 (ABCC13)

What is the basic structure and genetic organization of human ABCC13?

Human ABCC13 is an unusual truncated ABC transporter protein that spans approximately 70kb on human chromosome 21q11.2. The gene consists of 14 exons, and its open reading frame encodes a peptide of 325 amino acid residues. ABCC13's amino acid sequence, particularly in the membrane-spanning domains, exhibits remarkable similarity to other family members including ABCC1, ABCC2, ABCC3, and ABCC6 .

Structurally, ABCC13 differs from typical ABC transporters in its truncated nature, which has implications for its functional properties. The protein contains conserved regions that are characteristic of ABC transporters but lacks some domains typically found in fully functional transporters, suggesting it may have evolved specialized functions or regulatory roles.

The genomic organization of ABCC13 indicates evolutionary relationships with other ABC transporters, providing insights into the diversification of this important protein family throughout vertebrate evolution.

What is known about the expression patterns of ABCC13 in human tissues?

ABCC13 displays a distinctive tissue-specific expression pattern that suggests specialized physiological roles. The highest expression levels have been detected in fetal liver, indicating a potential developmental function . Additionally, ABCC13 is expressed in bone marrow, though its expression in peripheral blood leukocytes of adult humans is significantly lower, and no detectable levels have been observed in fully differentiated hematopoietic cells .

This expression profile suggests ABCC13 may be involved in early stages of hematopoiesis and cell differentiation. Supporting this hypothesis, studies have shown that ABCC13 expression in K562 cells decreases during cell differentiation induced by TPA (12-O-tetradecanoylphorbol-13-acetate) . This temporal regulation during differentiation further implicates ABCC13 in hematopoietic development processes.

How does ABCC13 differ structurally from other ABC transporters?

ABCC13 represents an atypical member of the ABC transporter family due to its truncated structure. While conventional ABC transporters typically feature two ATP-binding domains and two transmembrane domains, ABCC13's sequence suggests a non-standard configuration. The protein retains membrane-spanning domains with high similarity to other ABCC family members (ABCC1, ABCC2, ABCC3, and ABCC6), but its truncated nature raises questions about its transport capabilities .

To study these structural differences, researchers commonly employ sequence alignment tools to compare ABCC13 with other ABC transporters. This comparative approach helps identify conserved and divergent regions that may explain functional specialization. Structural prediction software and experimental techniques such as circular dichroism can provide insights into the protein's secondary and tertiary structure.

What approaches can be used to produce recombinant ABCC13 for research?

Production of recombinant ABCC13 requires careful consideration of expression systems and purification strategies. While the search results don't provide specific protocols for human ABCC13, similar approaches used for other ABC transporters can be adapted:

Expression Systems:

• Prokaryotic systems (E. coli): Suitable for producing specific domains of ABCC13 for structural studies, though may not provide proper folding for the full-length protein .

• Eukaryotic systems: Insect cells (Sf9, High Five) or mammalian cells (HEK293, CHO) are preferred for full-length membrane proteins to ensure proper folding and post-translational modifications.

Purification Strategy:

• Addition of affinity tags (His, GST) at either N- or C-terminus to facilitate purification

• Detergent solubilization of membrane fractions

• Sequential chromatography steps (affinity, ion exchange, size exclusion)

• Quality control through SDS-PAGE and Western blotting

Special consideration must be given to maintaining protein stability throughout the purification process, often requiring optimization of buffer conditions, addition of stabilizing agents, and working at lower temperatures.

What functional assays can determine if recombinant ABCC13 is properly folded and active?

Determining the proper folding and activity of recombinant ABCC13 presents unique challenges due to its truncated nature and incompletely understood natural substrates. Multiple complementary approaches can be used:

Structural Integrity Assays:

• Circular dichroism (CD) spectroscopy to assess secondary structure content

• Limited proteolysis to evaluate compactness and domain organization

• Thermal shift assays to determine protein stability

• Size exclusion chromatography to verify monodispersity

Functional Activity Assays:

• ATPase activity measurements (though these may be limited if ATP-binding domains are truncated)

• Substrate binding assays using potential transport substrates identified through homology with related transporters

• Reconstitution into liposomes or nanodiscs to test transport activity

• Fluorescence-based assays to monitor conformational changes upon substrate binding

Given the atypical nature of ABCC13, it's advisable to compare its properties with well-characterized ABC transporters as positive controls in these assays.

What evidence supports ABCC13's role in hematopoiesis and cell differentiation?

Multiple lines of evidence suggest ABCC13 plays a role in hematopoiesis and cell differentiation:

• Expression Pattern: ABCC13 shows highest expression in fetal liver and bone marrow, which are primary sites of hematopoiesis .

• Developmental Regulation: The protein exhibits higher expression in bone marrow compared to peripheral blood leukocytes, suggesting down-regulation during hematopoietic cell maturation .

• Differentiation-Associated Regulation: ABCC13 expression decreases during TPA-induced differentiation of K562 cells, indicating a potential role in maintaining an undifferentiated state .

To further investigate this role, researchers might employ the following methodological approaches:

• RNA interference or CRISPR-Cas9 gene editing to knock down/out ABCC13 in hematopoietic stem cells or precursor cell lines

• Colony-forming assays to assess effects on different hematopoietic lineages

• Flow cytometry to analyze changes in differentiation markers

• Transcriptome analysis to identify affected pathways

• ChIP-seq to determine transcription factor binding and epigenetic regulation of ABCC13 during differentiation

How might researchers investigate potential substrates of human ABCC13?

Investigating substrates for human ABCC13 is challenging given its truncated nature, which may affect typical transport functions. Nevertheless, several approaches can be employed:

Computational Methods:

• Homology modeling based on related transporters with known substrates

• Molecular docking simulations with candidate substrates

• Sequence analysis to identify conserved substrate-binding residues

Experimental Approaches:

• Direct binding assays using purified protein and labeled potential substrates

• Vesicle transport assays if transport function is preserved

• Cellular uptake/efflux studies in overexpression systems

• Metabolomics profiling of cells with manipulated ABCC13 expression

• Cross-linking experiments with photoactivatable substrate analogs

A strategic approach would begin with candidates known to interact with similar transporters (ABCC1, ABCC2, ABCC3, ABCC6) while considering the potentially altered selectivity due to ABCC13's truncated structure.

What can we learn from comparative studies between human ABCC13 and homologs in other species?

Comparative studies between human ABCC13 and homologs in other species can provide valuable insights into evolutionary conservation, functional specialization, and physiological roles. For example, research on wheat TaABCC13 has revealed important functions in phytic acid transport and heavy metal tolerance .

Research approaches for comparative studies:

• Phylogenetic Analysis:

  • Construction of evolutionary trees to determine when truncation occurred

  • Identification of conserved regulatory elements across species

• Functional Complementation:

  • Expression of human ABCC13 in model organisms lacking their native homolog

  • Assessment of rescue phenotypes to infer conserved functions

• Domain Swapping Experiments:

  • Creation of chimeric proteins between human ABCC13 and full-length homologs

  • Determination of functional domains through systematic domain exchanges

• Comparative Expression Analysis:

  • Tissue-specific expression patterns across species

  • Developmental regulation of expression

For instance, in wheat, TaABCC13 silencing resulted in reduced phytic acid content in grains and altered root development, particularly under cadmium stress . While plant and human ABCC13 likely have divergent functions, the plant studies demonstrate important principles about how ABCC transporters can affect development and stress responses that might inform human ABCC13 research directions.

What gene silencing approaches are most effective for studying ABCC13 function?

RNA interference (RNAi) and CRISPR-Cas9 technologies represent the primary approaches for silencing or knocking out ABCC13 to study its function. Based on studies in other systems like wheat TaABCC13 , the following methodological considerations are important:

RNAi Approaches:

• siRNA for transient knockdown in cell culture models

• shRNA for stable knockdown using lentiviral delivery systems

• Selection of target sequences with minimal off-target effects

• Validation of knockdown efficiency at both mRNA (qRT-PCR) and protein levels (Western blot)

CRISPR-Cas9 Approaches:

• Complete knockout via frameshift mutations in coding exons

• Conditional knockout using inducible or tissue-specific Cas9 expression

• CRISPRi for transcriptional repression without altering the genetic sequence

• Base editing or prime editing for specific amino acid changes

For studying ABCC13 specifically, researchers should consider:

• Targeting conserved regions to ensure efficient disruption of function

• Creating cell line panels with varying degrees of knockdown to study dose-dependent effects

• Using tissue-specific or inducible systems when studying developmental roles to avoid confounding effects

In wheat, RNAi silencing of TaABCC13 successfully reduced transcript levels in seeds and roots, allowing researchers to observe phenotypic effects including reduced phytic acid content in grains (22-34% reduction) and altered root development . Similar approaches could be adapted for studying human ABCC13 in appropriate cell types such as hematopoietic precursors or fetal liver cells.

What cell models are most appropriate for studying human ABCC13 function?

Given ABCC13's expression pattern and potential roles, the following cell models would be most appropriate for functional studies:

Primary Cell Models:

• CD34+ hematopoietic stem and progenitor cells (HSPCs) from bone marrow or cord blood

• Fetal liver cells (given high expression in fetal liver )

• Ex vivo expansion cultures of hematopoietic precursors

Cell Line Models:

• K562 cells (human myelogenous leukemia line that shows ABCC13 expression changes during differentiation )

• MOLM-13, THP-1, or other hematopoietic progenitor cell lines

• HEL (human erythroleukemia) cells for studying erythroid differentiation

Development and Differentiation Models:

• Inducible differentiation systems (e.g., TPA-induced differentiation of K562 cells )

• Embryoid bodies derived from human embryonic stem cells or induced pluripotent stem cells

• Organoid models of fetal liver development

Each model system offers distinct advantages:

• Cell lines provide consistency and ease of genetic manipulation

• Primary cells better reflect physiological expression and regulation patterns

• Differentiation models allow temporal analysis of ABCC13's role in development

The choice of model should be guided by the specific research question, with consideration for endogenous ABCC13 expression levels and the cellular context in which the protein naturally functions.

How can researchers overcome challenges in detecting endogenous ABCC13 expression?

Detection of endogenous ABCC13 expression can be challenging due to potentially low expression levels in many adult tissues and the lack of well-characterized antibodies. Researchers can employ several strategies to overcome these challenges:

RNA Detection Methods:

• Quantitative RT-PCR with carefully designed primers spanning exon junctions

• Digital droplet PCR for absolute quantification of low-abundance transcripts

• RNA in situ hybridization for spatial expression analysis in tissues

• Single-cell RNA sequencing to identify specific cell populations expressing ABCC13

Protein Detection Methods:

• Generation and validation of specific antibodies targeting unique ABCC13 epitopes

• Enrichment techniques (immunoprecipitation, subcellular fractionation) before Western blotting

• Proximity ligation assays for detecting protein-protein interactions

• Mass spectrometry-based proteomics with targeted acquisition methods

Reporter Systems:

• CRISPR knock-in of fluorescent tags or epitope tags at the endogenous ABCC13 locus

• Dual luciferase assays for studying promoter activity and gene regulation

• BAC transgenic approaches to maintain native genomic context and regulatory elements

When designing these approaches, researchers should consider:

• The possibly restricted expression of ABCC13 to specific developmental stages or tissues

• The potential for splice variants that might be missed by certain detection methods

• Cross-reactivity with more abundant ABC transporter family members

What potential roles might ABCC13 play in disease states or pathological conditions?

Given ABCC13's expression in hematopoietic tissues and potential role in cell differentiation, several disease-related research directions merit investigation:

Hematological Malignancies:

• Altered expression in leukemias, particularly those involving blocked differentiation

• Potential contribution to chemotherapy resistance through unknown transport functions

• Dysregulation in myelodysplastic syndromes or bone marrow failure conditions

Developmental Disorders:

• Potential involvement in Down syndrome pathophysiology due to its location on chromosome 21

• Impact of ABCC13 variants on fetal development or hematopoiesis

Comparative Disease Models:

• Insights from plant ABCC13 research suggesting roles in stress responses and toxin management

• Potential parallel functions in handling cellular stressors or environmental toxins

Research approaches might include:

• Genomic and transcriptomic analyses of patient samples

• Correlation of expression levels with clinical outcomes or treatment responses

• Functional studies using patient-derived cells

• Development of model systems that recapitulate disease-associated variations in ABCC13

The truncated nature of ABCC13 also raises the possibility that it may function as a regulatory protein rather than an active transporter, potentially modulating the activity of other transporters or cellular processes through protein-protein interactions.

How might systems biology approaches advance our understanding of ABCC13 function?

Systems biology approaches can help contextualize ABCC13 within broader cellular and physiological networks, potentially revealing functions that might be missed by reductionist approaches:

Network Analysis Methods:

• Protein-protein interaction mapping using proximity labeling techniques (BioID, APEX)

• Co-expression network analysis to identify genes with similar expression patterns

• Pathway enrichment analysis of genes affected by ABCC13 manipulation

Multi-omics Integration:

• Combined analysis of transcriptomics, proteomics, and metabolomics data after ABCC13 perturbation

• Correlation of ABCC13 expression with epigenetic modifications across developmental stages

• Integration with publicly available datasets to identify conditions that affect ABCC13 expression

Mathematical Modeling:

• Kinetic modeling of potential transport activities

• Gene regulatory network models of hematopoietic differentiation including ABCC13

• Evolutionary models to explain the persistence of a truncated transporter

These approaches may be particularly valuable for ABCC13 research given its unusual structure and incompletely understood function. For example, systems-level analysis could reveal whether ABCC13 functions as a competitor, regulator, or cofactor for other ABC transporters, potentially explaining its evolutionary conservation despite its truncated nature.

What are the key considerations for designing experiments to investigate ABCC13 transport activity?

Investigating ABCC13 transport activity presents unique challenges due to its truncated nature. Researchers should consider the following experimental design elements:

Substrate Selection:

• Test known substrates of related transporters (ABCC1, ABCC2, ABCC3, ABCC6)

• Include physiologically relevant molecules found in hematopoietic tissues

• Consider developmental signaling molecules given ABCC13's expression in fetal liver

Experimental Systems:

• Inside-out membrane vesicles from cells overexpressing ABCC13

• Reconstituted proteoliposomes with purified protein

• Whole-cell transport assays with fluorescent or radiolabeled substrates

• ABCC13-expressing Xenopus oocytes for electrophysiological measurements

Controls and Validation:

• Include functional ABC transporters as positive controls

• Use transport-deficient mutants as negative controls

• Employ specific inhibitors to confirm that observed activity is ABCC13-dependent

• Validate findings across multiple experimental systems

Given ABCC13's sequence similarity to ABCC1, ABCC2, ABCC3, and ABCC6 , researchers might begin by testing substrates known to be transported by these related proteins, while acknowledging that ABCC13's truncated structure may result in altered substrate specificity or transport mechanism.

How can researchers develop more specific tools for ABCC13 research?

Development of specific research tools is crucial for advancing ABCC13 research:

Antibody Development:

• Generation of monoclonal antibodies against unique epitopes of human ABCC13

• Validation across multiple techniques (Western blot, immunohistochemistry, flow cytometry)

• Creation of antibodies that distinguish between potential splice variants

Genetic Tools:

• CRISPR knock-in cell lines with endogenous tags (fluorescent, epitope)

• Inducible expression systems with physiologically relevant expression levels

• Domain-specific deletion constructs to map functional regions

Chemical Probes:

• Development of ABCC13-specific inhibitors through structure-based design

• Photoaffinity labels for identifying binding partners

• Fluorescent substrate analogs for real-time transport assays

Computational Resources:

• Homology models based on solved structures of related ABC transporters

• Prediction algorithms for identifying potential regulatory elements in the ABCC13 gene

• Databases integrating ABCC13 expression data across tissues and developmental stages