Protein phosphorylation and activation of platelets by wheat germ agglutinin.

The incorporation of 32Pi into the 47 and 20 kDa polypeptides in platelets activated by wheat germ agglutinin (WGA) was studied. The pattern of enhanced phosphorylation produced by the lectin was comparable to that by thrombin. The 47 kDa polypeptide was phosphorylated at both serine and threonine while the 20 kDa protein was mainly labeled at serine residues. However, the ratio of phosphoserine to phosphothreonine in the 47 kDa polypeptide in WGA-activated platelets was higher than thrombin-stimulated platelets. Addition of N-acetylglucosamine at different times blocked platelet activation by WGA. There was a concomitant modification in the phosphorylation of the 47 kDa protein. These data suggest that the phosphorylation of the 47 kDa polypeptide may modulate the WGA-receptor mediated activation of platelets. Our studies also demonstrate that activation of platelets by different stimuli may lead to differential phosphorylation of different amino acid residues in the same protein.