Abstract
Two proteins stimulating the GTPase activity of the smg -21 GTP-binding protein ( smg p21) having the same effector domain as the ras proteins ( ras p21s) are partially purified from the cytosol fraction of human platelets. These proteins, designated as smg p21 GTPase activating protein (GAP) 1 and 2, do not stimulate the GTPase activity of c-Ha- ras p21. The GAP activity for c-Ha- ras p21 is also detected in the cytosol fraction of human platelets. smg p21 GAP1 and 2 are separated from c-Ha- ras p21 GAP by column chromatographies. The activity of smg p21 GAP1 and 2 is killed by tryptic digestion or heat boiling. The Mr values of smg p21 GAP1 and 2 are similar and are estimated to be 2.5–3.5 × 10 5 by gel filtration analysis. These results indicate that there are two GAPs for smg p21 in addition to a GAP for c-Ha- ras p21 in human platelets.
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