A pleckstrin homology domain specific for phosphatidylinositol 4,5- bisphosphate (PtdIns-4,5-P₂) and fused to green fluorescent protein identifies plasma membrane PtdIns-4,5-P₂ as being important in exocytosis

Kinetically distinct steps can be distinguished in the secretory response from neuroendocrine cells with slow ATP-dependent priming steps preceding the triggering of exocytosis by Ca²⁺. One of these priming steps involves the maintenance of phosphatidylinositol 4,5-bisphosphate (PtdIns- 4,5-P₂) through lipid kinases and is responsible for at least 70% of the ATP-dependent secretion observed in digitonin-permeabilized chromaffin cells. PtdIns-4,5-P₂ is usually thought to reside on the plasma membrane. However, because phosphatidylinositol 4-kinase is an integral chromaffin granule membrane protein, PtdIns-4,5-P₂ important in exocytosis may reside on the chromaffin granule membrane. In the present study we have investigated the localization of PtdIns-4,5-P₂ that is involved in exocytosis by transiently expressing in chromaffin cells a pleckstrin homology (PH) domain that specifically binds PtdIns4,5-P₂ and is fused to green fluorescent protein (GFP). The PH-GFP protein predominantly associated with the plasma membrane in chromaffin cells without any detectable association with chromaffin granules. Rhodamine-neomycin, which also binds to PtdIns-4,5-P₂, showed a similar subcellular localization. The transiently expressed PH-GFP inhibited exocytosis as measured by both biochemical and electrophysiological techniques. The results indicate that the inhibition was at a step after Ca²⁺ entry and suggest that plasma membrane PtdIns-4,5-P₂ is important for exocytosis. Expression of PH-GFP also reduced calcium currents, raising the possibility that PtdIns-4,5-P₂ in some manner alters calcium channel function in chromaffin cells.