Protein Purification SOP for Cryo-EM

General guidance for purifying a sample destined for cryo-EM grids. Every construct is different — treat this as a starting checklist, not a substitute for construct-specific optimization.

1. Expression & lysis

• Expression temperature is host-dependent: for E. coli, lower temperature (16–18 °C) favors soluble, well-folded protein over yield; mammalian, insect, and yeast hosts each have their own standard ranges — follow host-specific practice rather than defaulting to the E. coli rule.
• Calculate the protein's theoretical pI and choose a lysis/SEC buffer pH shifted at least ~1 unit away from it (not simply "physiological") to keep net charge high enough to discourage aggregation.
• For intracellular proteins with multiple surface-exposed cysteines, include a reducing agent (e.g. DTT or TCEP) throughout purification to prevent disulfide-linked cysteine multimers from forming.
• Include protease inhibitors and, for complexes, a stabilizing ligand/cofactor if one is known.
• Clarify lysate by centrifugation at the highest speed your rotor allows before affinity capture.

2. Affinity capture & tag removal

• Keep affinity binding/wash steps short and cold (4 °C) to limit degradation and aggregation.
• Remove purification tags with a specific protease when the tag could interfere with particle alignment or introduce flexible, disordered mass.
• Confirm cleavage efficiency by SDS-PAGE before proceeding to polishing.

3. Size-exclusion polishing

• If the target still co-purifies with nucleic acid, aggregates, or close-molecular-weight contaminants after affinity capture, add an ion-exchange step (choosing cation vs. anion exchange based on the pI/buffer pH above) before the final SEC polish.
• Run a final SEC step in freshly filtered (0.22 µm), degassed buffer — take grids from the peak fraction, not a pool.
• A symmetric, single SEC peak at the expected size is a good sign; shoulders or a void-volume peak indicate aggregation or heterogeneity to resolve first.
• Favor simple buffers (e.g. HEPES or Tris, NaCl, low glycerol) — high-viscosity additives and detergents above their CMC can thicken ice and hurt particle distribution.

4. Quality checks before freezing grids

• Check monodispersity by dynamic light scattering or negative-stain EM immediately before grid prep.
• Target a concentration in the range typically needed for single-particle cryo-EM (often ~0.5–2 mg/mL for a mid-size complex) and screen a short dilution series on grids.
• Use the sample within hours of the final polishing step where possible; avoid repeated freeze–thaw cycles.

5. Handling & storage

• Keep the sample on ice throughout handling; avoid room-temperature dead time before plunge-freezing.
• If storage is unavoidable, snap-freeze small aliquots in liquid nitrogen rather than holding at 4 °C for more than a day.
• Record buffer composition, concentration, and SEC trace with the grid batch for later troubleshooting.