Avidin

Avidin is a tetrameric biotin binding protein (KD ~1 x 10-14) extracted from egg white. It prevents biotin and biotinylated proteins from binding to Strep-Tactin(R) or Strep-Tactin(R)XT, whereas binding of the Twin-Strep-tag(R) and Strep-tag(R)II is not influenced. Biotin and/or biotinylated proteins can potentially reduce the binding capacity of Strep-Tactin(R) as well as Strep-Tactin(R)XT and are present in cell lysates and cell culture media, especially from mammalian or insect cells. Thus, if Twin-Strep-tag(R) and Strep-tag(R)II fusion proteins from biotin-containing extracts or media are intended to be purified via Strep-Tactin(R) or Strep-Tactin(R)XT affinity chromatography, biotin has to be masked by addition of avidin prior to application onto the resin.