Binding of MBP-GPVI constructs to Fyn or Lyn SH3 domains in vitro. We measured the binding of MBP-GPVI to the following: GST-Fyn-SH3 (A), GST-Lyn-SH3 (B), and GST-Btk-SH3 (C) as a negative control, each attached to preblocked glutathione-coated microtiter plates. The MBP-GPVI constructs are as follows: QH, QN, LH, and LN (see legend to A). Bars and vertical lines represent the mean and 1 SD for each dataset (n ≥ 3 for each set). Binding was measured in the absence of divalent cations (■), 0.1 μM calcium (▩), or 0.3 μM calcium (□). The binding of QH and LH to Fyn (A) or Lyn (B) remained largely unaffected, whereas the binding of QN and LN was significantly diminished in the presence of calcium. Asterisks above a dataset designate a significant reduction with P < .01 relative to QH (wild-type GPVIa).