Partial isolation from intact cells of a cell surface-exposed lysophosphatidylinositol-phospholipase C

A novel cell surface phosphoinositide-cleaving phospholipase C (ecto- PLC) activity was isolated from cultured cells by exploiting its presumed external exposure. Biotinylation of intact cells followed by solubilization of the biotinylated proteins from a membrane fraction and recovery onto immobilized-avidin beads, allowed assay of this cell surface enzyme activity apart from the background of the substantial family of intracellular PLCs. Several cell lines of differing ecto-PLC expression were examined as well as cells stably transfected to overexpress the glycosylphosphatidylinositol (GPI)-anchored protein human placental alkaline phosphatase (PLAP) as a cell surface enzyme marker. The resulting bead preparations from ecto-PLC positive cells possessed calcium dependent PLC activity with preference for lysophosphatidylinositol (lysoPI) rather than phosphatidylinositol (PI). The function of ecto-PLC of intact cells evidently is not to release GPI-anchored proteins at the cell surface, as no detectable Ca²⁺ -dependent release of overexpressed PLAP from ecto PLC-positive cells was observed. To investigate the cell surface linkage of the ecru PLC itself, intact cells were treated with bacterial PI-PLC to cleave simple GPI anchors, but no decrease in ecto- PLC activity was observed. High ionic strength washes of biotinylated membranes prior to the generation of bead preparations did not substantially reduce the lysoPI-PLC activity. The results verify that the ecto-PLC is truly cell surface-exposed, and unlike other members of the PLC family that are thought to be peripheral membrane proteins, this novel lysoPI-PLC is most likely a true membrane protein.