Expression and purification of BirA fusion proteins as followed by SDS-PAGE (12%). (A) The target protein was expressed as a thioredoxin fusion. Lane 1, cell lysate supernatant; Lane 2, cell lysate pellet (resuspended in 8 M urea); Lane 3, imidazole eluate (Ni-NTA resin purified protein); Lane 4, size-exclusion chromatography purified protein; Lane 5, protein standards. (B) The target protein was expressed as an MBP fusion. Lane 1, cell lysate supernatant; Lane 2, cell lysate pellet (resuspended in 8 M urea); Lane 3, affinity and size-exclusion chromatography purified fusion protein; Lane 4, protein standards.