THE YEAST AND MAMMALIAN ISOFORMS OF PHOSPHATIDYLINOSITOL TRANSFER PROTEIN CAN ALL RESTORE PHOSPHOLIPASE C-MEDIATED INOSITOL LIPID SIGNALINGIN CYTOSOL-DEPLETED RBL-2H3 AND HL-60 CELLS

The mammalian phosphatidylinositol transfer proteins (PITP) and the ye ast Saccharomyces cerevisiae PITP (SEC14p) that show no sequence homol ogy both catalyze exchange of phosphatidylinositol (PI) between membra nes compartments in vitro. In HL-60 cells where the cytosolic proteins are depleted by permeabilization, exogenously added PITP alpha is req uired to restore G protein-mediated phospholipase C beta (PLC beta) si gnaling, Recently, a second mammalian PITP beta form has been describe d that shows 77% identity to rat PITP alpha, We have examined the abil ity of the two mammalian PITPs and SEC14p to restore PLC-mediated sign aling in cytosol-depleted HL-60 and RBL-2H3 cells. Both PITP alpha and PITP beta isoforms as well as SEC14p restore G protein-mediated PLC b eta signaling with a similar potency, In RBL-2H3 cells, crosslinking o f the IgE receptor by antigen stimulates inositol lipid hydrolysis by tyrosine phosphorylation of PLC gamma 1, Permeabilization of RBL cells leads to loss of PLC gamma 1 as well as PITP into the extracellular m edium and this coincides with loss of antigen-stimulated lipid hydroly sis. Both PLC gamma 1 and PITP were required to restore inositol lipid signaling, We conclude that (i) because the PI binding/transfer activ ities of PITP/SEC14p is the common feature shared by all three transfe r proteins, it must be the relevant activity that determines their abi lities to restore inositol lipid-mediated signaling and (ii) PITP is a general requirement for inositol lipid hydrolysis regardless of how a nd which isoform of PLC is activated by the appropriate agonist.