The second active site of ERp57 is critical for platelet aggregation. (A-D) Effect of preincubating human platelets with the ERp57 mutants (ss-oo or oo-ss). Wild-type (ss-ss) or completely inactivated ERp57 (oo-oo) were used as controls. Submaximal aggregation (baseline) was stimulated with convulxin (A) or thrombin (C). Representative tracings from a single experiment show the effect of adding ERp57 (2 µM) 10 minutes before the addition of the agonist. Panels B,D are the corresponding cumulative data ± SE (n = 3). There were no statistically significant differences between oo-oo and ss-oo or between oo-ss and ss-ss for convulxin or thrombin.