A novel acidic form of the phosphatidylinositol transfer protein is preferentially retained in permeabilized Swiss mouse 3T3 fibroblasts.

By use of indirect immunofluorescence it was shown that phophatidylinositol transfer protein (PI-TP) remains associated with the Golgi system of Swiss mouse 3T3 fibroblasts after permeabilization with streptolysin O. By Western blot analysis it was demonstrated that intact cells contain the phosphatidylinositol-bound form of PI-TP (pI 5.5) and a novel more acidic form of PI-TP (pI 5.4) in approximately equal amounts. After permeabilization, about 50% of the PI-TP was retained in the cells with an enrichment of the pH 5.4 form relative to the pH 5.5 form; the opposite was observed for the PI-TP released into the medium. Subfractionation of cell homogenates by centrifugation provided evidence that a distinct amount of PI-TP is strongly bound to the membrane fraction with the pH 5.4 form more prominently present than the pH 5.5 form.