Expression, purification, and size characterization of the FlPV-1 E7 C terminus. (A) Expression and purification. The last 59 C-terminal residues of FlPV-1 E7 were fused to the C terminus of MBP and overexpressed in bacteria (lane 1). Pellets were sonicated, and the centrifugation-cleared supernatant was purified by affinity on MBP-specific amylose resin (lanes 2, flowthrough; lane 3, maltose eluate) and subjected to TEV digestion, yielding MBP and the FlPV-1 E7 C terminus (lane 4). The FlPV-1 E7 C terminus was then separated from MBP and TEV by gel filtration (lane 5). Due to its small size (6.5 kDa) and its high concentration, the sodium dodecyl sulfate-denatured construct was visualized in the migration front of the gel. (B) Preparative gel filtration. Upon preparative gel filtration, the FlPV-1 E7 C terminus (theoretical size of monomer, 6.5 kDa) shows an elution peak corresponding to a 13-kDa dimer, preceding the peak of the FlPV-1 E6 monomer (9.2 kDa). mAU, milli-absorbance units.