(a) Initial velocity (Vi in µM/min) is measured for the wild-type Itk enzyme and the Itk(BtkSH3) enzyme at three different enzyme concentrations, 0.12 µM, 1.2 µM and 1.6 µM, using Biotin labeled peptide-B as a substrate. At each concentration the wild type Itk data are shown in black and the Itk(BtkSH3) data are in grey. Experiment was carried out in duplicate and the data shown are representative of three independent experiments. (b) Full-length wild-type Itk (lane 1) or Itk(BtkSH3) (lane 2) were co-expressed with PLCγ1 in Sf9 insect cells. After three days, cell lysates were prepared, and immunoblotted for phospho-PLCγ1 (top panel), total PLCγ1 (middle panel) or Itk (bottom panel). The bands on the western blot are quantified using Quantity One software and setting wild-type Itk to one.