Phospholipase D regulation and localisation is dependent upon a phosphatidylinositol 4,5-bisphosphate-specific PH domain

The signalling pathway leading, for example, to actin cytoskeletal reorganisation, secretion or superoxide generation involves phospholipase D (PLD)-catalysed hydrolysis of phosphatidylcholine to generate phosphatidic acid, which appears to mediate the messenger functions of this pathway [1-3]. Two PLD genes (PLD1 and PLD2) with similar domain structures have been cloned and progress has been made in identifying the protein regulators of PLD1 activation, for example Arf and Rho family members [4,5], The activities of both PLD isoforms are dependent on phosphatidylinositol 4,5-bisphosphate Pl(4,5)P-2 and our sequence analysis suggested the presence of a pleckstrin homology (PH) domain in PLD1, although its absence has also been claimed [4], Investigation of the inositide dependence showed that a bis-phosphorylated lipid with a vicinal pair of phosphates was required for PLD1 activity. Furthermore, PLD1 bound specifically and with high affinity to lipid surfaces containing Pl(4,5)P-2 independently of the substrate phosphatidylcholine, suggesting a key role for the PH domain in PLD function. Importantly, a glutathione-S-transferase (GST) fusion protein comprising GST and the PH domain of PLD1 (GST-PLD1-PH) also bound specifically to supported lipid monolayers containing Pl(4,5)P-2. Point mutations within the PLD1 PH domain inhibited enzyme activity, whereas deletion of the domain both inhibited enzyme activity and disrupted normal PLD1 localisation, Thus, the functional PH domain regulates PLD by mediating its interaction with polyphosphoinositide-containing membranes; this might also induce a conformational change, thereby regulating catalytic activity.