Abstract
We found that FcγRII‐mediated cell spreading and phagocytosis were correlated with an increase of phosphatidylinositol 4,5‐bisphosphate [PI(4,5)P~2~] level in cells. During the spreading, a long‐lasting elevation of PI(4,5)P~2~ and concomitant actin polymerization occurred. Filopodia and lamellae of spreading cells were enriched in phosphatidylinositol 4‐phosphate 5‐kinase Iα (PIP5‐kinase Iα) that colocalized with PI(4,5)P~2~ and actin filaments. Both spreading and phagocytosis were inhibited by expression of the C~374–440~ fragment of PIP5‐kinase Iα or the pleckstrin homology domain of phospholipase Cδ~1~ (PLCδ~1~‐PH), two probes binding PI(4,5)P~2~. These probes reduced the amount of PI(4,5)P~2~ in the cells, evoked reorganization of the actin cytoskeleton and abolished PI(4,5)P~2~ elevation during phagocytosis. Simultaneously, PLCδ~1~‐PH‐GFP reduced the amount of PIP5‐kinase Iα associated with the plasma membrane. In vitro studies demonstrated that PIP5‐kinase Iα‐GST bound PI(4,5)P~2~, phosphatidylinositol 4‐monophosphate, and less efficiently, phosphatidic acid. The data suggest that the PLCδ~1~‐PH domain, and possibly also the C~374–440~ fragment, when expressed in cells, can compete with endogenous PIP5‐kinase Iα for PI(4,5)P~2~ binding in the plasma membrane leading eventually to PI(4,5)P~2~ depletion.