Filtration of the cell lysate

Centrifugation does not remove all of the cell debris so a tangential flow ultrafiltration step with a 300 kDa membrane follows. The retentate is collected. Instead of this device you can also use a simple dead-end sterile filter (0.22 µm poresize) but it is easier the way described above (the dead-end filter gets stucked after some time...).

With the cleared lysate the histidine affinity tag comes into play - want to know how?.