Synaptojanin inhibition of phospholipase d activity by hydrolysis of phosphatidylinositoi, 4.5-bisphosphate

A 150 kDa protein that inhibits PLD activity stimulated by ARF and PI(4.5)P₂was previously purified from rat brain. We now identify thei PLD inhibitor as synaptojanin nerve tedrminal protein implicated in the encocytosis of synaptic vesicles and shown to be and inositol polyphosphate 5 phosphatase We found that synaptojanin removes one phosphate group from 1(1.4.5)P but none from either 1(1.3)Unlike other members of this enzyme family however, it removes both the 4 and 5-phosphates of P(4,5)P₂ and the 4-phosphate of Mg²⁺is required for the hydrolysys of 1(1,4,5)P by synaptojanin. as are other known 5-phosphatases. but it is unnecessary for that of phosphoinositides. These differences in enzymatic properties show that synaptojanin is a unique member among the diverse 5 phosphatase family the inhibition of PLD by synaptojanin is attributable to its ability to dephosphry late P1(4,5)P₂It did not inhibit PLD without PI(4,5)P₂and the extent of PLD inhibition was related to the extent of PI(4,5)P₂ hydrolysis in sub-trate vesicles. It hsa been proposed that PI(4,5)P₂ biosynthesis and PLD activation by ARF constitute a positive loop to increased the amounts of PI(4,5)P₂amd [jps[jatodoe acod (PA) during membrane vesiculation The PA thus produced together with PI(4,5)P₂ facilitates vesicle coating. PI(4,5)P₂hydrolysis and consequent PLD inhibiton by synaptojanin might constitute a mechanism to halt the positive loop connecting these phospholipids and serve as a signal for uncoating.