CD33 lectin activity and serine phosphorylation interrelate.

(A) Effect of protein phosphorylation manipulation on RBC rosette formation. Working in a 24-well cluster tray, CD33-transfected COS cells were treated with either staurosporine (S, 2 μM), BIM (B, 2 μM), both staurosporine and BIM (S + B), or okadaic acid (OA, 0.4 μM) for 60 minutes, or with PMA (18 nM) for 12 minutes, all at 37° C. Rosettes were then allowed to form and were fixed, stained with AEC, and counted. Duplicate wells were counted, and the average ± SD shown (P < .01). Decrease of CD33 phosphorylation leads to an increase in lectin binding of RBCs; conversely, an increase in CD33 phosphorylation leads to a lowered rosetting ability. (B) Rosetting of RBCs with CD33T₂₉₉Δ. COS cells transiently transfected with pCD33T₂₉₉Δ rosette RBCs in the standard rosetting assay. Treatment with either PMA or staurosporine plus BIM (S + B) is without effect. Results of representative experiments are shown.