The A4 peptide decreases the amount of filamin A coimmunoprecipitating with GpIbα. The A4 peptide, when it is added to platelet lysates simultaneously with the GpIbα-specific antibody AN51, decreases the amount of immunodetectable filamin A that coimmunoprecipitates with GpIbα (bars present means + SDs; ** P < .01 when compared to 0 A4 peptide by Student t test).