Intracellular cleavage of glycosylphosphatidylinositol by phospholipase D induces activation of protein kinase C alpha

Many proteins are anchored to the cell membrane by glycosylphosphatidylinos itol (GPI). One of the functions proposed for the GPI anchor is as a possib le mediator in signal transduction through its hydrolysis. GPI-specific pho spholipase D (GPI-PLD) is a secretory protein that is suggested to be invol ved in the release of GPI-anchored protein from the membrane. In the presen t study we examined how GPI-PLD is involved in signal transduction, When in troduced exogenously and overexpressed in cells, GPI-PLD cleaved the GPI an chors in the early secretory pathway, possibly in the endoplasmic reticulum , resulting in an increased production of diacylglycerol, Experiments in vi tro and in vivo showed that the association of protein kinase C alpha (PKC alpha) with membranes was increased markedly by expression of GPI- PLD in c ells. Furthermore, sucrose-density-gradient centrifugation and immunofluore scence microscopy demonstrated that PKC alpha was translocated to the endop lasmic reticulum membrane in cells expressing GPI-PLD, in contrast with its association with the plasma membrane in cells treated with PMA. We also co nfirmed that the phosphorylation of c-Fos as well as PKC alpha itself was g reatly enhanced by the expression of GPI-PLD. Taken together, these results suggest that GPI-PLD is involved in intracellular cleavage of the GPI anch or, which is a new potential source of diacylglycerol production to activat e PKC alpha.