Phosphorylation of Synaptic GTPase Activating Protein (synGAP) by Ca^(2+)/calmodulin-dependent protein kinase II (CaMKII) and cyclin-dependent kinase 5 (CDK5) alters the ratio of its GAP activity toward Ras and Rap GTPases

Abstract

SynGAP is a neuron-specific Ras and Rap GTPase-activating protein (GAP) found in high concentration in the postsynaptic density (PSD) fraction from mammalian forebrain. We have previously shown that, in situ in the PSD fraction or in recombinant form in Sf9 cell membranes, synGAP is phosphorylated by Ca^(2+)/calmodulin-dependent protein kinase II (CaMKII), another prominent component of the PSD. Here we show that recombinant synGAP (r-synGAP), lacking 102 residues at the N-terminus, can be purified in soluble form and is phosphorylated by cyclin-dependent kinase 5 (CDK5) as well as by CaMKII. Phos-phorylation of r-synGAP by CaMKII increases its HRas GAP activity by 25% and its Rap1 GAP activity by 76%. Conversely, phosphorylation by CDK5 increases r-synGAPs HRas GAP activity by 98% and its Rap1 GAP activity by 20%. Thus, phosphorylation by both kinases increases synGAP activity, but CaMKII shifts the relative GAP activity toward inactivation of Rap1; whereas CDK5 shifts the relative activity toward inactivation of HRas. GAP activity toward Rap2 is not altered by phosphorylation by either kinase. CDK5 phosphorylates synGAP primarily at two sites, S773 and S802. Phosphorylation at S773 inhibits r-synGAP activity, whereas phosphorylation at S802 increases it. However, the net effect of concurrent phosphorylation of both sites, S773 and S802, is an increase in GAP activity. SynGAP is phosphorylated at S773 and S802 in the PSD fraction, and its phosphorylation by CDK5 and CaMKII is differentially regulated by activation of NMDA-type glutamate receptors in cultured neurons.

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