SARS-CoV-2 Spike Protein: From Sequence to Assay-Grade Antigen in 18 Days | AtaGenix

The Challenge

The SARS-CoV-2 spike protein is a 1,273-amino-acid homotrimer with 22 N-linked glycosylation sites per monomer. Its receptor-binding domain (RBD) must fold correctly to present the ACE2-binding epitope that most diagnostic antibodies target. The client had tested an E. coli-expressed RBD fragment, but it lacked glycosylation, showed poor ELISA signal-to-noise ratios, and failed to capture patient serum antibodies consistently. They needed a full-length, properly folded spike protein — fast.

AtaGenix's Approach

AtaGenix deployed its HEK293 transient expression platform with a codon-optimized spike construct containing a C-terminal His-tag and a furin cleavage site mutation (to stabilize the prefusion conformation). Key steps included:

• Day 1–3: Gene synthesis, vector construction, and sequence verification
• Day 4–10: HEK293 transient transfection at 1 L scale; culture supernatant harvested at day 6 post-transfection
• Day 11–15: Two-step purification: Ni-NTA affinity capture followed by preparative SEC to isolate the trimeric peak
• Day 16–18: QC panel (SDS-PAGE, Western Blot, analytical SEC, endotoxin LAL assay) and lyophilized shipment

Purification Results

The SDS-PAGE gel below shows the full purification workflow. Strong, clean bands in elution fractions E1–E8 confirm efficient target capture with minimal co-purifying contaminants:

Figure 1. SDS-PAGE analysis of SARS-CoV-2 spike protein purification. MW = marker; IN = input; FT = flow-through; W1–W2 = wash fractions; E1–E12 = elution fractions. Strongest bands appear in E1–E8, confirming high-yield recovery with clean background.

Outcome

The purified spike protein showed >95% trimer content by analytical SEC, with endotoxin below 0.1 EU/µg. When benchmarked against the client's previous E. coli-derived RBD, the mammalian-expressed full-length spike achieved a 3.5-fold improvement in ELISA signal-to-noise ratio for patient serum IgG detection. The client proceeded to validate their serological assay kit using AtaGenix-supplied antigen as the capture reagent.

This case study represents an anonymized project outcome. Results may vary depending on target protein, construct design, and project scope. All client information is subject to NDA. Specific timelines, yields, and QC specifications should be confirmed during project consultation.