Recent evidence has implicated caveolae/DIGs in various aspects of signal transduction, a process in which
polyphosphoinositides play a central role. We therefore undertook a study to determine the distribution of
phosphoinositides and the
enzymes that utilize them in these
detergent-insoluble domains. We report here that the
polyphosphoinositide phosphatase, but not several other
phosphoinositide-utilizing
enzymes, is highly enriched in a low density, Triton-insoluble membrane fraction that contains
caveolin. This fraction is also enriched in
polyphosphoinositides, containing approximately one-fifth of the total cellular
phosphatidylinositol (4,5)P2. Treatment of cells with the
tumor-promoting
phorbol ester,
phorbol 12-myristate 13-acetate (PMA), did not alter the distribution of
polyphosphoinositides or the
polyphosphoinositide phosphatase. However, PMA treatment did lead to a decrease in the
mitogen-activated protein kinase and actin present in these domains. PMA also induced the recruitment of
protein kinase C alpha to the caveolae/DIGs fraction. These findings suggest that
polyphosphoinositides, the
polyphosphoinositide phosphatase and
protein kinase C play an important role in the structure or function of
detergent-insoluble membrane domains.