Recombinant Human Transmembrane protein C9orf91 (C9orf91)

What is C9orf91/TMEM268 and what are its fundamental structural characteristics?

Transmembrane protein 268 (TMEM268), also known as C9orf91, is a human protein encoded by the TMEM268 gene located on chromosome 9 at position 9q32. The protein consists of 342 amino acids and features eight alternative splice variants . TMEM268 contains two predicted transmembrane regions located at amino acids 104-125 and 130-152 . It has an isoelectric point of 5.19 and a molecular weight of approximately 37.6 kDa .

The protein belongs to a domain of unknown function (DUF4481), which spans amino acids 37-328 . BLAST analysis has shown no paralogs within humans, zebrafish, or fruit flies, suggesting unique functional characteristics . The variant NP_694590.2 is the most extensively studied form of this protein .

What methodologies are recommended for detecting endogenous C9orf91 in cellular systems?

For detecting endogenous C9orf91 in cellular systems, immunological techniques have proven most effective. Immunocytochemistry (ICC) has been successfully employed using antibodies such as ab121527 at 0.25 μg/mL concentration in human cell lines like U-2 OS (human bone osteosarcoma epithelial cells) . For tissue samples, immunohistochemistry (IHC) on formalin/PFA-fixed paraffin-embedded sections has been validated at 1/500 dilution, with heat-mediated antigen retrieval using citrate buffer (pH 6) .

For protein expression analysis, the following approach is recommended:

• Cell/tissue lysis with appropriate buffers (RIPA for general purposes)

• Protein quantification (Bradford or BCA assay)

• SDS-PAGE separation followed by western blotting

• Probing with validated anti-C9orf91 antibodies

What is known about the biological function of C9orf91/TMEM268?

C9orf91/TMEM268 has been functionally characterized as a protein that stabilizes cell surface expression of ITGAM (Integrin Alpha M) and participates in the adhesion and migration of phagocytes during bacterial clearance . This suggests a potential role in immune response and cellular migration processes.

The protein has evolutionary conservation from fruit flies to primates, with no expression detected in organisms simpler than insects . This evolutionary pattern suggests the emergence of its function coincided with the development of more complex organisms, potentially relating to specialized cellular processes in higher eukaryotes.

What is the genomic context and expression pattern of C9orf91?

The TMEM268 gene maps to chromosome 9 at position 9q32, with neighboring genes DFNB31 and ATP6V1G1 . This genomic context may provide insights into potential co-regulation or functional relationships.

Based on immunohistochemistry studies, expression has been detected in various human tissues including:

• Cerebellum (showing specific staining patterns)

• Prostate tissue

• Various cell lines including U-2 OS

Mass spectrometry-based proteomics has identified C9orf91 in normal human tissues and lung cancer samples, suggesting differential expression patterns that may be clinically relevant .

What are the optimal conditions for expression and purification of recombinant C9orf91?

For recombinant expression of C9orf91, the following protocol is recommended based on standard practices for transmembrane proteins:

Expression System Selection:

• Mammalian expression systems (HEK293 or CHO cells) are preferred for proper folding and post-translational modifications

• Bacterial systems (E. coli) may be suitable for specific domains but typically not for the full-length protein due to transmembrane regions

Purification Strategy:

• Add an affinity tag (His6, FLAG, or GST) to either the N- or C-terminus, avoiding disruption of transmembrane domains

• Use mild detergents for solubilization (DDM, CHAPS, or digitonin at 0.5-1%)

• Perform affinity chromatography using appropriate resins

• Consider size exclusion chromatography as a final purification step

Critical Parameters:

• Buffer composition: 20-50 mM Tris or phosphate buffer (pH 7.4-8.0), 150-300 mM NaCl

• Addition of glycerol (10-15%) to improve stability

• Temperature control: maintain at 4°C during purification

• Consider adding protease inhibitors to prevent degradation

How can researchers effectively investigate the DUF4481 domain in C9orf91?

The DUF4481 domain (Domain of Unknown Function) spans amino acids 37-328 in C9orf91 . To characterize this domain, researchers should consider:

Domain Characterization Approach:

• Structure Prediction Analysis:

  • Employ bioinformatics tools (AlphaFold, I-TASSER) for structural prediction

  • Use molecular dynamics simulations to explore conformational flexibility

• Functional Mutation Studies:

  • Design alanine scanning mutations across conserved residues

  • Create domain truncation constructs to assess function

  • Employ CRISPR/Cas9 to generate domain-specific mutations in cellular models

• Binding Partner Identification:

  • Perform pull-down assays with the isolated domain

  • Use yeast two-hybrid screening or BioID proximity labeling

  • Conduct cross-linking mass spectrometry (XL-MS) to identify interacting proteins

• Evolutionary Analysis:

  • Compare domain conservation across species

  • Identify potential functional motifs through multiple sequence alignment

What approaches can be used to study C9orf91's role in phagocyte adhesion and migration?

Given C9orf91's role in stabilizing cell surface expression of ITGAM and participating in phagocyte adhesion and migration , the following methodological approaches are recommended:

Functional Assays:

• Adhesion Assays:

  • Plate-based adhesion to extracellular matrix components

  • Flow chamber assays to measure adhesion under shear stress

  • Quantification using fluorescent labeling or impedance-based systems

• Migration Assays:

  • Transwell migration assays with chemoattractants

  • Wound healing/scratch assays for directional migration

  • Time-lapse microscopy with cell tracking analysis

• Phagocytosis Assays:

  • Fluorescent bacterial uptake assays

  • Live cell imaging of phagocytic cup formation

  • Quantification of bacterial clearance in C9orf91-depleted cells

Mechanistic Studies:

• Co-immunoprecipitation with ITGAM to confirm direct interaction

• Surface biotinylation assays to measure ITGAM surface expression

• FACS analysis to quantify integrin expression and activation states

• Phosphorylation profiling of downstream signaling pathways

How can researchers identify and validate SNPs and mutations in C9orf91?

For identification and validation of Single Nucleotide Polymorphisms (SNPs) and mutations in C9orf91, researchers should consider:

Identification Methods:

• Genomic Sequencing:

  • Targeted sequencing of the C9orf91 locus

  • Whole exome sequencing with focus on chromosome 9q32

  • Analysis using variant calling pipelines

• Proteogenomic Approaches:

  • Mass spectrometry analysis of unassigned tandem mass spectra

  • Custom database searches incorporating known SNP variants

  • Validation at 1% false discovery rate (FDR)

Validation Methods:

• Functional Impact Assessment:

  • Site-directed mutagenesis to introduce identified variants

  • Expression of mutant constructs in relevant cell models

  • Assays to measure changes in protein stability, localization, and function

• Population Analysis:

  • Frequency analysis across different ethnic groups

  • Association studies with disease phenotypes

  • Linkage analysis in familial studies

Table 1: Reported SNPs in C9orf91 from Proteogenomic Analysis

What methodologies are most effective for studying the transmembrane topology of C9orf91?

Given that C9orf91 has two predicted transmembrane regions (amino acids 104-125 and 130-152) , the following approaches are recommended to study its membrane topology:

Experimental Approaches:

• Protease Protection Assays:

  • Treatment of membrane preparations with proteases

  • Analysis of protected fragments by western blotting

  • Identification of cytoplasmic vs. extracellular regions

• Glycosylation Mapping:

  • Introduction of N-glycosylation sites at various positions

  • Assessment of glycosylation status to determine luminal/extracellular regions

  • Endoglycosidase H treatment to confirm glycosylation

• Cysteine Scanning Mutagenesis:

  • Introduction of cysteine residues throughout the protein

  • Treatment with membrane-permeable or impermeable thiol-reactive reagents

  • Detection of modified residues to map topology

• Fluorescence-Based Methods:

  • GFP fusion constructs at different termini

  • pH-sensitive fluorescent protein tags (pHluorin)

  • FRET analysis of protein orientation

Computational Methods:

• Use of transmembrane prediction algorithms (TMHMM, HMMTOP, Phobius)

• Integration of hydrophobicity analysis with evolutionary conservation

• Structural modeling with membrane positioning consideration

How does C9orf91 interact with ITGAM and what experimental approaches best characterize this interaction?

The interaction between C9orf91 and ITGAM (Integrin Alpha M) is crucial for understanding its role in phagocyte function . To characterize this interaction:

Interaction Mapping:

• Co-immunoprecipitation Studies:

  • Reciprocal pulldowns with antibodies against both proteins

  • Analysis under different cell activation states

  • Detection of potential co-precipitating complex members

• Domain Mapping:

  • Generation of deletion constructs to identify interaction domains

  • Peptide array screening to identify specific binding motifs

  • Mutagenesis of key residues to disrupt interaction

• Proximity Labeling:

  • BioID or TurboID fusion proteins for in vivo proximity labeling

  • APEX2-based labeling in subcellular compartments

  • Analysis of labeled proteins by mass spectrometry

Functional Characterization:

• Surface Expression Analysis:

  • Flow cytometry to quantify ITGAM surface levels

  • Biotinylation assays with C9orf91 knockdown/overexpression

  • Pulse-chase experiments to assess protein stability

• Localization Studies:

  • Co-localization analysis by immunofluorescence

  • Live-cell imaging with fluorescently tagged proteins

  • Super-resolution microscopy to determine nanoscale organization

What are the most effective protocols for studying differential expression of C9orf91 in disease contexts?

To investigate differential expression of C9orf91 in various disease contexts, particularly in cancer:

Expression Analysis Techniques:

• Transcriptional Profiling:

  • qRT-PCR for targeted analysis of C9orf91 expression

  • RNA-Seq to assess expression in relation to other genes

  • Analysis of alternative splicing patterns across tissues

• Protein Level Analysis:

  • Western blotting with quantitative densitometry

  • Immunohistochemistry with tissue microarrays

  • Mass spectrometry-based proteomics for unbiased quantification

Disease Association Studies:

• Cancer Tissue Analysis:

  • Comparison between matched normal and tumor samples

  • Correlation with clinical parameters and patient outcomes

  • Integration with mutation data from cancer genome databases

• Functional Impact Assessment:

  • Knockdown/overexpression studies in relevant cell lines

  • Analysis of effects on cell proliferation, migration, and invasion

  • Pathway analysis to identify affected signaling networks

Table 2: C9orf91 Expression in Different Tissues Based on Proteomics Studies

What quality control measures should be implemented when working with recombinant C9orf91?

When working with recombinant C9orf91, the following quality control measures are essential:

Protein Quality Assessment:

• Purity Verification:

  • SDS-PAGE analysis with Coomassie staining (>90% purity recommended)

  • Western blotting with specific antibodies

  • Mass spectrometry-based identification

• Functionality Testing:

  • Binding assays with known interactors (e.g., ITGAM)

  • Circular dichroism to assess secondary structure

  • Limited proteolysis to evaluate folding quality

• Storage Stability Assessment:

  • Testing protein activity after different storage conditions

  • Freeze-thaw stability analysis

  • Aggregation monitoring by dynamic light scattering

Experimental Validation:

• Use positive and negative controls in all functional assays

• Include wild-type protein alongside mutant variants

• Verify expression construct sequences prior to protein production

• Test multiple batches to ensure reproducibility

How can researchers effectively study the role of C9orf91 in different cellular contexts?

To investigate C9orf91 function across different cellular contexts:

Genetic Manipulation Strategies:

• CRISPR/Cas9 Gene Editing:

  • Complete knockout of C9orf91

  • Introduction of specific mutations or tagged versions

  • Conditional deletion using Cre-loxP or similar systems

• RNA Interference:

  • siRNA for transient knockdown experiments

  • shRNA for stable knockdown cell lines

  • Inducible knockdown systems for temporal control

• Overexpression Systems:

  • Stable cell lines with controlled expression levels

  • Inducible expression systems (Tet-On/Tet-Off)

  • Viral delivery methods for difficult-to-transfect cells

Functional Assessment Approaches:

• Cell Type-Specific Analysis:

  • Compare effects in phagocytic vs. non-phagocytic cells

  • Assess function in primary cells vs. immortalized lines

  • Evaluate in differentiated vs. undifferentiated states

• High-Content Screening:

  • Automated microscopy to assess phenotypic changes

  • Multi-parameter analysis of cellular responses

  • Machine learning-based feature extraction

By implementing these methodological approaches, researchers can comprehensively investigate the structure, function, and disease associations of C9orf91/TMEM268, advancing our understanding of this transmembrane protein in normal physiology and pathological conditions.