PHOSPHATIDYLINOSITOL TRANSFER PROTEIN IN MURINE EMBRYONAL CARCINOMA-CELLS DURING RETINOIC ACID-INDUCED DIFFERENTIATION

Phosphatidylinositol transfer protein (Pl-TP) was studied in P19 embry onal carcinoma (EC) cells at different stages of retinoic acid (RA) in duced differentiation. Western blot analysis indicated an increased ex pression of PI-TP (35 kDa) during differentiation. Western blots of is oelectric focusing gels showed that the 35 kDa band consisted of the P I-carrying form of PI-TP (pl 5.5) and of a novel, more acidic form of PI-TP (pl 5.4), levels of both of which increased during differentiati on. These increased levels were not reflected in the in vitro PI-trans fer activity of the cytosolic fraction nor in the mRNA levels as analy zed by northern blotting. By using indirect immunofluorescence it was shown that PI-TP is localized in the cytoplasm and associated with per inuclear Golgi structures and that this distribution is slightly affec ted during RA-induced differentiation. Immunoprecipitation of PI-TP fr om [P-32]P-i labeled cells demonstrated that the level of phosphorylat ion of PI-TP is high in undifferentiated P19 EC cells and low after 5 days of RA-induced differentiation. These results strongly suggest tha t changes in the levels of PI-TP are intimately connected with changes in the growth characteristics of P19 EC cells during RA-induced diffe rentiation. It remains to be established to what extent this connectio n is governed by the recent finding that PI-TP is an essential cytosol ic factor in stimulating phospholipase C activity.