Author: Sara Lindgren

### Purpose

The protocol describes how to prepare selenomethionyl (Se-Met) protein using a regular E.coli strain. Selenium can be used for phase determination in [multi-wavelength anomalous diffraction (MAD) method](http://brie.bmsc.washington.edu/scatter/). Se-Met can often replace methionine residues in a protein without affecting the protein's properties, therefore producing a protein advantageous for crystal structure solving. Also, the X-ray absorption edge of selenium is easily accessible by synchrotron radiation, making a Se-Met crystal ideal for collecting anomalous X-ray diffraction data. The Se-Met proteins can also be prepared using insect cells and CHO cells, which will be described in separate protocols.

### Materials

1. LB media
- antibiotics (1000x conc.)
- 1M IPTG
- M9 media (minimal media)
- 1 Liter 5x M9 media: (sterile filtered)
  - 30g Na2HPO4 or 64g Na2HPO4-7H2O
  - 15g KH2PO4
  - 5g NH4Cl
  - 2.5g NaCl
     - Dilute and autoclave before use.
- Amino acid 50x stock
       - Use all amino acids EXCEPT Gly, Ala, Pro, Asn, Cys, and Met at a concentration of 2mg/ml
       - To help in dissolving the amino acids, autoclave for 10 minutes.
- 20% glucose (sterile filtered or autoclaved)
- 1M MgSO4 (sterile filtered)
- 2M CaCl2 (sterile filtered)
- 0.5% (w/v) Thiamine Solution (sterile filtered)


### Procedure

**Day 1**

1. Prepare a 2mL day culture consisting of 2mL LB media, 2uL antibiotics (1000x conc.), and a single E.coli colony. Grow at 37°C all day.
- Prepare M9 stock media. Dilute and autoclave before use.
- Prepare amino acid 50x Stock.
- Prepare a 150mL overnight culture consisting of 150mL LB, 150uL antibiotics (1000x conc.), and 150uL of day culture. Grow at 37°C overnight.

**Day 2**

1. To each liter M9 (1x conc.) add:
  - 10mL 20% Glucose (sterile filtered or autoclaved)
  - 2mL 1M MgSO4 (sterile filtered)
  - 0.05mL 2M CaCl2 (sterile filtered)
  - 0.1mL 0.5% (w/v) thiamine solution (sterile filtered)
  - 1mL antibiotics (1000x conc.)
  - 20mL amino acid 50x Stock (If precipitate is seen, heat to 60-70°C and shake.)
- Inoculate M9 with 50mL overnight culture and grow until an OD600=0.5-0.6. (~2.0 - 2.5 hours)
- Add 100mg threonine, lysine hydrochloride, phenylalanine to the culture.
- Add 50mg leucine, isoleucine, valine to the culture (all as solid powders).
- Add 120mg DL-Se-Met or 60mg L-Se-Met to the culture (as a solid powder).
- Continue to grow the culture for 15 minutes.
- Induce with 1mL 1M IPTG (final concentration = 1mM).
- Grow about 6-8 hours (whatever is optimal for the protein of interest).
- Collect cells as usual and proceed to purification steps.


### Expected Results

- Se-Met protein will show slightly larger MW than the native protein in mass spectrum.
- Se-Met protein may behave slightly differently from the native protein in purifications and crystallization.


### References


1. Doublie, S. (1997) Preparation of Selenomethionyl Proteins for Phase Determination. *Methods in Enzymology* 276, 523-530.
- Deacon, AM., Ealick SE. (1999) Selenium-based MAD phasing: setting the sites on larger structures. *Structure* 7, R161-R166
- Protocol originally obtained from Qing Fan at Don Wiley's Lab.
- [X-ray Anomalous Scattering](http://brie.bmsc.washington.edu/scatter/): Principles, WebTools, and Related Links provided by Ethan A. Merritt at University of Washington.